soc: silabs: Only initialize HFXO Manager if HFXO is enabled

Only initialize the HFXO Manager HAL driver if the HFXO is enabled in
DeviceTree, the device uses SYSRTC for timekeeping, and Power Manager
is enabled. HFXO Manager integrates with the Sleeptimer HAL driver for
SYSRTC to autonomously wake the HFXO prior to Sleeptimer wakeup from
deep sleep. It is not needed on devices that don't have HFXO-SYSRTC
integration, and it is not needed if the application doesn't use deep
sleep.

Add missing call to init_hardware() prior to init().

Signed-off-by: Aksel Skauge Mellbye <aksel.mellbye@silabs.com>