pinctrl: sh-pfc: r8a7796: Add voltage switch operations for SDHI

This patch supports the {get,set}_io_voltage operations of SDHI.

This operates the POCCTRL0 register on R8A7796 SoC and makes 1.8v/3.3v
voltage switch.

Based on work by Takeshi Kihara and Wolfram Sang.

Cc: Takeshi Kihara <takeshi.kihara.df@renesas.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>
Reviewed-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>