arch: arm: Set Zero Latency IRQ to priority level zero:

Set Zero Latency IRQ to priority level zero and SVCs to priority level
one when Zero Latency IRQ is enabled.
This makes Zero Zatency truly zero latency when the kernel has been
configured with userspace enabled, or when IRQ offloading is used.

Exceptions can still delay Zero Latency IRQ, but this is considered
ok since exceptions indicate a serious error, and the system needs to
recover.

Fixes: #7869

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