pinctrl: sh-pfc: r8a7795: Support none GPIO pins with configurable drive-strength

There are pins on the r8a7795 which are not part of a GPIO bank nor
can be muxed between different functions. They do however allow for the
drive-strength to be configured. Add those pins to the list of pins and
to the drive-strength configuration registers.

The pins can now be referred to in DT by function names and their
drive-strength modified.
Signed-off-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>