pinctrl: sh-pfc: refactor voltage setting

All known hardware being able to switch voltages has the same POCCTRL
register. So, factor out the common code to the core and keep only
the pin-to-bit mapping SoC specific. Convert the only user, r8a7790.
In case POCCTRL should ever get more complex (more voltages to select?),
we should probably switch over to a describing array like drive strength
does currently.

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>