pinctrl: sh-pfc: r8a7794: Implement voltage switching for SDHI

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

Based on work by Wolfram Sang for the r8a7790.

Signed-off-by: Simon Horman <horms+renesas@verge.net.au>
Acked-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>