arch: arm: Fix zero interrupt latency priority level

Change the zero latency interrupt priority level from 2 to 1.
This is the priority level that the kernel has reserved for the
zero latency IRQ feature by the _IRQ_PRIO_OFFSET constant.
The zero latency IRQ will now not be masked by the irq_lock function.

Update comments to reflect the priority levels reserved by the kernel.

Fixes: #8073

Signed-off-by: Joakim Andersson <joakim.andersson@nordicsemi.no>
3 files changed