nrf52: call nRFCrypto.begin()/end() only once

[fdlamotte]

This follows discussion from #2824

Still think this has to be checkout profundly to asses the hw is used as it should (this is a touchy thing)

Needs some tests on different boards too

don't know if there is a difference energy wise

[fdlamotte]

for RNG we should probably use the work already made in #2280 already uses HW on most architectures ...