ARM: sun5i: Add UART2 pin group

There's one UART2 pin group that can be used across all sun5i SoCs.
However, the A10s already has one pin group for that controller.

Change the index of the one in the A10s DTSI, and add the common one to
sun5i.dtsi

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