ft4222: Add documentation for usb platform

Signed-off-by: Brendan Le Foll <brendan.le.foll@intel.com>

README.md

@@ -30,6 +30,10 @@ ARM
 * [Bannana Pi](../master/docs/banana_pi.md)
 * [Beaglebone Black](../master/docs/beaglebone.md)
 
+USB
+---
+* [FT4222](../master/docs/ftdi_ft4222.md)
+
 Installing on your board
 ========
 

docs/ftdi_ft4222.md

@@ -0,0 +1,23 @@
+FTDI FT4222H                             {#ft4222}
+============
+
+The FT4222H is a High/Full Speed USB2.0-to-Quad SPI/I2C device controller. Mraa
+supports it as a USB subplatform using the libft4222 library from FTDI which
+can be found
+[here](http://www.ftdichip.com/Support/SoftwareExamples/libft4222-1.2.0.240.tgz).
+
+The FT 4222H has 4 configuration modes selected by {DCNF1, DCNF0}.  The c hip
+configuration mode will determine the number of USB interface s for data stream
+s and for GPIOs control. Mraa supports only chip CNFMODE0.
+
+Whilst mraa can support custom board in CNFMODE0 (support for other mode is
+welcome!) there may be some work to be done. We test using FTDI's UNFT4222EV
+reference board. More detail on this board can be found
+[here](http://www.ftdichip.com/Support/Documents/DataSheets/Modules/DS_UMFT4222EV.pdf).
+
+Interface notes
+---------------
+
+You will need to unload all ftdi kernel modules for libft4222 to work
+correctly. You will also have to compile mraa with FT4222 support which may not
+be enabled by default.

docs/index.md

@@ -46,6 +46,7 @@ Specific platform information for supported platforms is documented here:
 - @ref bananapi
 - @ref beaglebone
 - @ref nuc5
+- @ref ft4222
 
 ## DEBUGGING