pinctrl: sh-pfc: r8a7790: Break out USB0 OVC/VBUS

Create a new group for the USB0 OVC/VBUS pin by itself. This
allows us to monitor PWEN as GPIO on the Lager board.

Signed-off-by: Magnus Damm <damm@opensource.se>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>