pinctrl: sh-pfc: r8a7791: Group USB PWEN and OVC pins together

This groups USB PWEN and OVC pins together on R8A7791 SoC,
the same way it's done on R8A7790, since both are needed
for a USB device.

Signed-off-by: Valentine Barshak <valentine.barshak@cogentembedded.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>