pinctrl: meson: meson8: add support for the Meson8m2 SoC

Add Meson8m2 support to the existing Meson8 pinctrl driver. Since there
are only very few changes (Meson8m2 has an extra signal on 10 CBUS pins,
no other differences were found so far).
Add the new compatible strings for Meson8m2 to the existing Meson8
driver so we don't have to duplicate the whole driver.

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Reviewed-by: Kevin Hilman <khilman@baylibre.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>