pinctrl: sh-pfc: r8a7795: Add drive strength support

Define the drive strength registers for the R8A7795. As the PFC driver
for the SoC only defines GPIO pins at the moment, limit drive strength
support to those pins. Pins without GPIO capabilities will be supported
later.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>