USB: android: Add support to get usb-core-id using devicetree

Android gadget driver is binded with UDC using usb-core-id value.
This change adds support to pass usb-core-id value through devicetree.
This is useful where multiple USB cores are used in device mode
concurrently and it is required to bind each gadget driver with each
core specific UDC.

Change-Id: Iba22bfbccb4f2a87a9ea513e074694004523933e
Signed-off-by: Mayank Rana <mrana@codeaurora.org>