soc: silabs_exx32: Make HFRCO frequency configurable

This commit adds an option to configure the frequency
in Hz for the HFRCO clock source. The default value
for this option is 0 which skips the configuration of the HFRCO.
This feature is supported for efm32gg, efm32jg, efm32pg, efm32fg
and efm32mg SOCs currently.

Signed-off-by: Pooja Karanjekar <pooja.karanjekar@lemonbeat.com>