Update to nrfx - fixes UART issue that consumes power even when shutdown

[furbrain]

This fixes a bug in the previous nrfx implementation that failed to fully shut down the UART. This lead to a
sleep current consumption of ~1mA.

My initial attempt to fix this #8089 somehow broke the USB as described in #8391, so it was reverted.

This takes code from a later version of the nrfx firmware.

[dhalbert]

Thanks for this!

I meant to say I will test this first.

[dhalbert]

Tested with test from #8391. Works -- Does not exhibit the problem described there.

Thanks!