pinctrl: meson: fix meson8b ao pull register bits

AO pull register definition is inverted between pull (up/down) and
pull enable. Fixing this allows to properly apply bias setting
through pinconf

Fixes: 0fefcb68 ("pinctrl: Add support for Meson8b")
Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>