pinctrl: sh-pfc: r8a77470: Add SDHI support

Add SH_PFC_PIN_CFG_IO_VOLTAGE definition for the SDHI pins
capable of switching voltage, also add pin groups and functions
for SDHI0 and SDHI1. Please note that with the RZ/G1C only 1
bit of the POC Control Register is used to control each interface.

Signed-off-by: Fabrizio Castro <fabrizio.castro@bp.renesas.com>
Reviewed-by: Biju Das <biju.das@bp.renesas.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>