HSI: Add HSI ABI documentation

Adds sysfs HSI framework documentation
Signed-off-by: NCarlos Chinea <carlos.chinea@nokia.com>
Acked-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>
Acked-by: NLinus Walleij <linus.walleij@linaro.org>

Documentation/ABI/testing/sysfs-bus-hsi

+What:		/sys/bus/hsi
+Date:		April 2012
+KernelVersion:	3.4
+Contact:	Carlos Chinea <carlos.chinea@nokia.com>
+Description:
+		High Speed Synchronous Serial Interface (HSI) is a
+		serial interface mainly used for connecting application
+		engines (APE) with cellular modem engines (CMT) in cellular
+		handsets.
+		The bus will be populated with devices (hsi_clients) representing
+		the protocols available in the system. Bus drivers implement
+		those protocols.
+
+What:		/sys/bus/hsi/devices/.../modalias
+Date:		April 2012
+KernelVersion:	3.4
+Contact:	Carlos Chinea <carlos.chinea@nokia.com>
+Description:	Stores the same MODALIAS value emitted by uevent
+		Format: hsi:<hsi_client device name>