pinctrl: meson: get rid of pin_base

pin_base was used with the manually set pin offset in meson pinctrl. This
is no longer the case, pin_base is 0 on every meson pinctrl controllers
and should go away.

Tested-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>