ARM: dts: sun7i: Add uart3_pins_b pinctrl setting

The uart3_pins_a multiplexes the uart3 pins to port G, add a pinctrl entry
for mapping them to port H (as used on the Bananapi).

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>