board: ti: beagle_x15: added USB initializtion code

Implemented board_usb_init(), board_usb_cleanup() and
usb_gadget_handle_interrupts() in beagle_x15 board file that
can be invoked by various gadget drivers.

Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>