arm64: dts: berlin4ct: add default pinmux for uart0

Add urt0 txd and rxd muxing setup in the dtsi because uart0 always uses
them to work, no other possibilities.

Signed-off-by: Jisheng Zhang <jszhang@marvell.com>
Acked-by: Sebastian Hesselbarth <sebastian.hesselbarth@gmail.com>
Acked-by: Antoine Tenart <antoine.tenart@free-electrons.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Sebastian Hesselbarth <sebastian.hesselbarth@gmail.com>