HSI: Add HSI API documentation

Add an entry for HSI in the device-drivers section of
the kernel documentation.
Signed-off-by: NCarlos Chinea <carlos.chinea@nokia.com>

Documentation/DocBook/device-drivers.tmpl

@@ -437,4 +437,21 @@ X!Idrivers/video/console/fonts.c
 !Edrivers/i2c/i2c-core.c
   </chapter>
 
+  <chapter id="hsi">
+     <title>High Speed Synchronous Serial Interface (HSI)</title>
+
+     <para>
+	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.
+
+	HSI provides multiplexing for up to 16 logical channels,
+	low-latency and full duplex communication.
+     </para>
+
+!Iinclude/linux/hsi/hsi.h
+!Edrivers/hsi/hsi.c
+  </chapter>
+
 </book>