gpio: rcar: Add r8a7796 (R-Car M3-W) support

R-Car Gen3's GPIO blocks are identical to Gen2's in every respect.

Based on work for the r8a7795 (R-Car H3) by Ulrich Hecht.

Cc: Ulrich Hecht <ulrich.hecht+renesas@gmail.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Tested-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>