soc: arm: nrf53: workaround pop lr after wfi crash

On nRF5340 net core it was observed that when `wfi` instruction was
followed by `pop {r0, lr}` in the `arch_cpu_idle` function,
the value of `lr` sometimes got read as 0 from memory despite
having correct value stored in the memory.

This commit inserts additional `nop` instruction after waking up
to delay access to the memory.

Signed-off-by: Andrzej Kuroś <andrzej.kuros@nordicsemi.no>