soc: silabs: silabs_s2: Align power states with HAL

The definition of the EM3 energy mode is that software switches
off the LFRCO and LFXO before entering deep sleep. On Series 2,
oscillators are clocked on-demand based on peripheral requests
from hardware. Requesting EM3 will result in EM2 if any active
peripheral uses one of the oscillators, and requesting EM2
will result in EM3 if no peripherals use the oscillators.

In version 2025.6 of the HAL, this was reflected in the API
of the power manager by making EM3 an alias of EM2. Reflect
this in Zephyr by removing the separate EM3 power state.

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