Documentation: HSI: Add some general description for the HSI subsystem

Add a document, which gives a rough introduction about what HSI
is and how its handled by the Linux kernel.
Signed-off-by: NSebastian Reichel <sre@kernel.org>
Reviewed-by: NPavel Machek <pavel@ucw.cz>

Documentation/hsi.txt

+HSI - High-speed Synchronous Serial Interface
+
+1. Introduction
+~~~~~~~~~~~~~~~
+
+High Speed Syncronous Interface (HSI) is a fullduplex, low latency protocol,
+that is optimized for die-level interconnect between an Application Processor
+and a Baseband chipset. It has been specified by the MIPI alliance in 2003 and
+implemented by multiple vendors since then.
+
+The HSI interface supports full duplex communication over multiple channels
+(typically 8) and is capable of reaching speeds up to 200 Mbit/s.
+
+The serial protocol uses two signals, DATA and FLAG as combined data and clock
+signals and an additional READY signal for flow control. An additional WAKE
+signal can be used to wakeup the chips from standby modes. The signals are
+commonly prefixed by AC for signals going from the application die to the
+cellular die and CA for signals going the other way around.
+
++------------+                                 +---------------+
+|  Cellular  |                                 |  Application  |
+|    Die     |                                 |      Die      |
+|            | - - - - - - CAWAKE - - - - - - >|               |
+|           T|------------ CADATA ------------>|R              |
+|           X|------------ CAFLAG ------------>|X              |
+|            |<----------- ACREADY ------------|               |
+|            |                                 |               |
+|            |                                 |               |
+|            |< - - - - -  ACWAKE - - - - - - -|               |
+|           R|<----------- ACDATA -------------|T              |
+|           X|<----------- ACFLAG -------------|X              |
+|            |------------ CAREADY ----------->|               |
+|            |                                 |               |
+|            |                                 |               |
++------------+                                 +---------------+
+
+2. HSI Subsystem in Linux
+~~~~~~~~~~~~~~~~~~~~~~~~~
+
+In the Linux kernel the hsi subsystem is supposed to be used for HSI devices.
+The hsi subsystem contains drivers for hsi controllers including support for
+multi-port controllers and provides a generic API for using the HSI ports.
+
+It also contains HSI client drivers, which make use of the generic API to
+implement a protocol used on the HSI interface. These client drivers can
+use an arbitrary number of channels.
+
+3. hsi-char Device
+~~~~~~~~~~~~~~~~~~
+
+Each port automatically registers a generic client driver called hsi_char,
+which provides a charecter device for userspace representing the HSI port.
+It can be used to communicate via HSI from userspace. Userspace may
+configure the hsi_char device using the following ioctl commands:
+
+* HSC_RESET:
+ - flush the HSI port
+
+* HSC_SET_PM
+ - enable or disable the client.
+
+* HSC_SEND_BREAK
+ - send break
+
+* HSC_SET_RX
+ - set RX configuration
+
+* HSC_GET_RX
+ - get RX configuration
+
+* HSC_SET_TX
+ - set TX configuration
+
+* HSC_GET_TX
+ - get TX configuration