arm: exx32: Add Silabs EFM32JG12B soc files

The Silicon Labs EFM32 Jade Gecko MCU includes:

    * Cortex-M3 core at 40MHz
    * up to 1024KB of flash and 256KB of RAM
    * multiple low power peripherals

This is basically the same as the EFM32 Pearl Gecko, but with an ARM
Cortex-M3 core instead of a Cortex-M4F.

Signed-off-by: Christian Taedcke <christian.taedcke@lemonbeat.com>