pinctrl: sunxi: Introduce the strict flag

Our pinctrl device should have had strict set all along. However, it wasn't
the case, and most of our old device trees also have a pinctrl group in
addition to the GPIOs properties, which mean that we can't really turn it
on now.

All our new SoCs don't have that group, so we should still enable that mode
on the newer one though.

In order to enable it by default, add a flag that will allow to disable
that mode that should be set by pinctrl drivers that cannot be migrated.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>