ARM: dts: berlin: add the pinctrl node and muxing setup for uarts

Add pinctrl bindings and system control nodes to what we currently know
about Berlin SoCs. Where available, also set default pinctrl property
for uarts, when there is only one pinmux option for it.

Signed-off-by: Antoine Tenart <antoine.tenart@free-electrons.com>
Signed-off-by: Sebastian Hesselbarth <sebastian.hesselbarth@gmail.com>