usb: gadget: gr_udc: Use platform_get_irq instead of irq_of_parse_and_map

Use platform_get_irq as no mapping needs to be done. No functional difference
for SPARC which is the typical environment for the driver though. Suggested by
Mark Rutland.

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