zephyr: Kconfig: SOC_GECKO_CUSTOM_RADIO_PHY option for RAIL for proprietary

Currently on zephyr, RAIL library is used only by Bluetooth applications,
with this update, it will be able to be used for sample applications
for custom radio phys.
All files were copied from Silicon Labs GSDK v4.2.4.

Signed-off-by: Zoltan Havas <zoltan.havas@silabs.com>