drivers: entropy: nRF: clear NVIC pending before doing WFE on RNG IRQ

We need to clear the NVIC Pending bit for the RNG IRQ before
doing any WFEs and expect to wake up by RNG events. This is
because the event register will be set only if NVIC status
is changed from 0 to 1.

Signed-off-by: Ioannis Glaropoulos <Ioannis.Glaropoulos@nordicsemi.no>