NRF5x: rationalise sleep+critical

This follows up from previous closed PRs #8366 and #9211, in an attempt
to fix the issue originally reported as #5647.

As discussed in those PRs, the sleep behaviour of the NRF5x family is
apparently unsafe, at least for the cases where the SoftDevice is
disabled.

This commit fixes it up by:

* Collapsing down to 1 implementation, from 3.
* Using standard PRIMASK disable when SoftDevice is disabled.
* Using standard WFI when SoftDevice is disabled.

This commit shouldn't change the functionality for when it is enabled -
it retains the "SEVONPEND" and "FPU pending" fudges.

If SoftDevice is enabled, we continue to hope that its sd_app_evt_wait()
call does the Right Thing(TM). However, this seems unlikely, as
documentation and the "nosd" stubs visible suggests they just do a
"WFE", when we are looking for a "WFI" equivalent. Or it's possible
their real implementation does the unsafe and racy "SEV; WFE; WFE;"
sequence.

Comments added to clarify what we are hoping for from SoftDevice.

Author: kjbracey