pinctrl: sh-pfc: r8a7796: Add drive strength support

Define the drive strength registers for the R8A7796. Add pins which are
not part of a GPIO bank nor can be muxed between different functions but
which still allow for their drive-strength to be configured.
Signed-off-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>