boards: Add SiLabs SLWSTK6121A (WGM160P)

The WGM160P module combines the WF200 Wi-Fi transceiver with
an EFM32GG11 MCU
This code is based on the efm32gg_stk3701a board definitions

Signed-off-by: Thorvald Natvig <thorvald@natvig.com>
13 files changed