dts: efm32gg12b: add pin-controller binding

The gecko UART driver needs pinctrl support for SOC_GECKO_SERIES1
devices, this has been added to jg and pg 12b series in 40fa9650 but
is missing in others, causing some build failurse.

Add the device nodes for the gg11b and gg12b files since they contain
gecko-uart references and seems to be under the SERIES1 define.

Signed-off-by: Fabio Baltieri <fabiobaltieri@google.com>