pinctrl: sh-pfc: Add generic IOCTRL register description

Add a generic way to describe IOCTRL registers (for e.g. SD I/O voltage
and time delay control), like is already done for config, drive, and
bias registers.

This makes the sh-pfc core code aware of these registers, which will
ease introducing suspend/resume support later.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>