ARM: dts: sun7i: som204: Use the UART3 TX and RX pin group

The SOM204-EVB doesn't use the CTS pin, and thus was defining its own
pinctrl node for the UART3 muxing. Since we split away the TX and RX pin,
we can use the global node now, and only have the RTS pin in our local
node.

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