ARM: dts: sun7i: Add PA gpio pin to Wexler TAB7200 tablet

Add PA gpio pin for controlling power of two external amplifiers (NS4890) which are used on the Wexler TAB7200 tablet

Signed-off-by: Aleksei Mamlin <mamlinav@gmail.com>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>