sh-pfc: r8a7791: add MLB+ pin group

Add MLB+ 3-pin mode pin group to R8A7791 PFC driver.

Based on original patch by Andrey Gusakov <andrey.gusakov@cogentembedded.com>.

Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Acked-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>