Port Zephyr to Silabs EFM32ZG-STK3200

[zephyrbot]

Reported by Marcio Montenegro:

Add port for Silicon Labs EFM32 Zero Gecko. It is based on the ARMCortex-M0+ processor.
The starter kit is EFM32WG-STK3200
Also includes drivers for GPIO,UART
Related issue:
GH-3432

(Imported from Jira ZEP-2017)

[nashif]

no action for a while, just open a PR when you have this ready.