pinctrl: sunxi: Support I/O bias voltage setting on H6

H6 SoC has a "pio group withstand voltage mode" register (datasheet
description), that needs to be used to select either 1.8V or 3.3V I/O mode,
based on what voltage is powering the respective pin banks and is thus used
for I/O signals.

Add support for configuring this register according to the voltage of the
pin bank regulator (if enabled).

This is similar to the support for I/O bias voltage setting patch for A80
and the same concerns apply. See:

  commit 402bfb3c ("Support I/O bias voltage setting on A80")

Signed-off-by: Ondrej Jirman <megous@megous.com>
Acked-by: Maxime Ripard <maxime.ripard@bootlin.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>