pinctrl: sunxi: Prepare for alternative bias voltage setting methods

H6 has a different I/O voltage bias setting method than A80. Prepare
existing code for using alternative bias voltage setting methods.

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>