soc: silabs: add a config for low latency interrupt on Sliabs S2 devices

The high frequency clock was always restored before handling the
interrupts to make sure that the system clock is as expected. However,
the response time to interrupt when we were in EM2 was a between 300 and
600 us. By default, we use the low interrupt latency.

Signed-off-by: Bastien Beauchamp <bastien.beauchamp@silabs.com>