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 ...

[IoTThinks]

don't know if there is a difference energy wise

Let me test on RAK4631 and T096.
The original PR has little power impact. Therefore, the fix will likely have little power impact too.