usb: gadget: R8A66597 peripheral controller support.

While in-tree support for the R8A66597 host side has been supported for
some time, the peripheral side has so far been unsupported. This adds a
new USB gadget driver which bridges the gap and finally wires up the
peripheral side as well.

Signed-off-by: Yoshihiro Shimoda <shimoda.yoshihiro@renesas.com>
Tested-by: Magnus Damm <damm@igel.co.jp>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>