usb: gadget: Add UDC driver for Aeroflex Gaisler GRUSBDC

This adds an UDC driver for GRUSBDC USB Device Controller cores available in the
GRLIB VHDL IP core library. The driver only supports DMA mode.

Signed-off-by: Andreas Larsson <andreas@gaisler.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>