ARM: sunxi: dt: Add uart3 dt node

Both A10 and A13 Allwinner SoCs have a Synopsys APB uart3 device
available, so add it to the sunxi.dtsi file

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Acked-by: Emilio López <emilio@elopez.com.ar>