soc: arm: remove all unnecessary NMI_INIT() calls

NMI_INIT() is now a no-op, so remove it from all SoC code. Also remove
the irq lock/unlock pattern as it was likely a cause of copy&paste when
NMI_INIT() was called.

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>