soc: silabs: silabs_s2: Activate Zero Latency IRQ with level 2

silabs_s2 uses simplicity_sdk hal library, which already have by default
a zero latency IRQs mechanism with a hardcoded value. In order to be
aligned with simplicity_sdk, we need to activate Zero Latency IRQ in
Zephyr by default. The level (2) depends on the hardcoded
value in simplicity_sdk (CORE_ATOMIC_BASE_PRIORITY_LEVEL). Without this
fix, if you use an IRQ with a priority of 0 or 1, irq_lock() and
irq_unlock() have no effect for this IRQ.

Signed-off-by: Martin Hoff <martin.hoff@silabs.com>