soc/arm/silabs: support BLE with PM in Series 2 SoCs

Using EM2 or deeper sleep states (where HF clocks are off) requires
special care if BLE radio is used, since BLE radio relies on that clock,
and its power/clock requirements need to be taken into account
On SiLabs, radio PM is implemented as part of RAIL blob, which relies
on sl_power_manager HAL service. I've implemented SoC PM
state changes using sl_power_manager instead of emlib, and added
call to RAIL PM initialization in Gecko HCI driver.

Signed-off-by: Roman Dobrodii <rdobrodii@antmicro.com>