USB: Separate out android usb gadget devicetree documentation

Android USB gadget driver is having its own device in devicetree
and now its property is growing in size. Hence creates its own
devicetree documentation file.

Change-Id: I0336ec484192bcb70a18c44dae2eb5553a66b6d2
Signed-off-by: Mayank Rana <mrana@codeaurora.org>