pinctrl: meson: remove unused pin_base

While removing the need to have pin_base defined in meson pinctrl
drivers, I forgot to remove the corresponding field from the
pinctrl_data structure.

Fixing this now.

Fixes: 70e5ecb1 ("pinctrl: meson: get rid of pin_base")
Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
Reviewed-by: Kevin Hilman <khilman@baylibre.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>