soc/arm/silabs_exx32: fix PM implementation - wake up using BURTC timer

- Add Gecko BURTC sys_clock driver to handle wake up from EM2,3 states
- Remove custom PM policy and dependency on HAL sl_power_manager service
- EM1 supported in all configurations
- EM2,3 supported only if SysTick is replaced by BURTC

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