pinctrl: sh-pfc: Implement pinconf power-source param for voltage switching

The pfc in the R8A7790 (and probably others in the R-Car gen 2 family)
supports switching SDHI signals between 3.3V and 1.8V nominal voltage,
and the SD driver should do that when switching to and from UHS modes.

Add a flag for pins that have configurable I/O voltage and SoC
operations to get and set the nominal voltage.  Implement the pinconf
power-source parameter using these operations.

Signed-off-by: Ben Hutchings <ben.hutchings@codethink.co.uk>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>