ext: Add Silabs Gecko SDK for EFR32MG12P SoCs

Origin: Silicon Labs EFR32MG12P Mighty Gecko Series Device Support
CMSIS PACK
URL: https://www.silabs.com/documents/public/cmsis-packs/SiliconLabs.EFR32MG12P_DFP.5.5.0.pack
Version: 5.5.0 (2018-05-07)
Purpose: Add support for Silicon Labs EFR32MG12P SoCs
License: Zlib
Maintained-by: External

To update the Gecko SDK, download the current version from the given URL
rename .pack to .zip, unpack and replace the following folder:
* Device/SiliconLabs/ should replace
  ext/hal/silabs/gecko/Device/SiliconLabs

Signed-off-by: Diego Sueiro <diego.sueiro@gmail.com>
78 files changed