ARM: dts: sun7i: Add uart4_pins_b definition

Some boards (e.g. the BananaPro) use alternative pins for uart4, add a pinmux
entry for these.

Signed-off-by: Michael Ring <mail@michael-ring.org>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>