pinctrl: sh-pfc: r8a7790: Implement voltage switching for SDHI

All the SHDIs can operate with either 3.3V or 1.8V signals, depending
on negotiation with the card.

Implement the {get,set}_io_voltage operations and set the related
capability flag for the associated pins.

Signed-off-by: Ben Hutchings <ben.hutchings@codethink.co.uk>
Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Cc: linux-gpio@vger.kernel.org
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>