Add USB gadget support for port YC301 on the KFR2R09 board.

The r8a66597-udc driver is hooked up as a platform device,
clocks are enabled via I2C and some registers are configured
to enable the USB in gadget mode. The hardware driving the
USB port is the on-chip USB0 block in the sh7724 processor
configured as USB gadget controller.

This board is using external hardware to detect USB hotplug
events and allows the processor to dynamically start and stop
clocks. This well thought out hardware feature is unused at
this point and plug and play is unfortunately unsupported.
To properly support all hardware features the USB gadget
stack may need some adjustment.
Signed-off-by: Magnus Damm <damm@igel.co.jp>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>