ARM: dts: sun7i: add pin muxing options for UART2

UART2 is used on CubieTruck to connect to the Bluetooth module.
Add the pin set used in this case.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>