pinctrl: meson: add optional region for drive strength

On the G12a, there is a new 'region' to handle the drive-strength.
This is optional since the older do not have this.

Fixes: 29ae0952 ("pinctrl: meson-g12a: add pinctrl driver support")
Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>