usb: gadget: Create supported function list dynamically

If android_usb platform driver passes list of supported
function, create the list of supported functions dynamically.
This allows to create the list specific to a particular target
and avoid allocating and initializing memory for un-needed
function drivers.

Change-Id: I48a67d55265a909a7c711edc7dbb24f9b1830ad2
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>