pinctrl: renesas: Add I/O voltage level flag

This patch adds config macros describing the voltage levels available on
a pin. The current default (3.3V/1.8V) maps to zero to avoid having to
change existing PFC implementations.

Signed-off-by: Ulrich Hecht <uli+renesas@fpond.eu>
Tested-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Link: https://lore.kernel.org/r/20210112165912.30876-3-uli+renesas@fpond.eu


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