pinctrl: sh-pfc: r8a7794: add USB pin groups

Add USB0/1 pin groups to R8A7794 PFC driver.

Signed-off-by: Shinobu Uehara <shinobu.uehara.xc@renesas.com>
[Sergei: rebased, renamed, added changelog.]
Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>