pinctrl: sh-pfc: r8a77970: Fix pin I/O voltage control support

I've included the pin I/O voltage control into the R8A77970 PFC driver but
it was incomplete because:
- SH_PFC_PIN_CFG_IO_VOLTAGE pin flags weren't set properly;
- sh_pfc_soc_info::ioctrl_regs wasn't set at all...

Fixes: b92ac66a ("pinctrl: sh-pfc: Add R8A77970 PFC support")
Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>