boards: arm: add initial support for efr32bg22_brd4184b

This adds basic support for 'B' revision of the Silicon Labs EFR32BG
BRD4184 (Thunderboard BG22) board. Due to missing drivers, some of the
on-board peripherals are currently unsupported:

- Silicon Labs Si7021 relative humidity and temperature sensor
- Vishay VEML6035 ambient light sensor
- Knowles SPK0641HT4H-1 MEMS microphones
- TDK InvenSense ICM-20648 6-axis inertial sensor

Signed-off-by: Piotr Dymacz <pepe2k@gmail.com>