cpu/nrf5x: implement pm_off() for nRF53/9160

[dylad]

Contribution description

This PR implements pm_off() support for nRF9160 and nRF53.
The register moved to the regulator peripheral compared to nRF52.
Nordic also have two different names for the same bit of this register, so I add a small workaround. (Could have use '1' instead but I prefer to clearly identify bits within registers).

Testing procedure

Run tests/periph/pm on nrf9160dk or nrf5340dk-app
and use pm off command

Issues/PRs references

None.

[riot-ci]

Murdock results

✔️ PASSED

7aa7e79 cpu/nrf5x: implement pm_off() for nRF53/9160

Success	Failures	Total	Runtime
10065	0	10066	13m:46s

Artifacts

• Documentation preview

[benpicco]

Want to take the opportuninty to untangle that #ifdef mess? It's really hard to read.

How about

#if defined(REGULATORS_SYSTEMOFF_SYSTEMOFF_Enter)
    NRF_REGULATORS_S->SYSTEMOFF = REGULATORS_SYSTEMOFF_SYSTEMOFF_Enter;
#elif defined(CPU_FAM_NRF51)
    NRF_POWER->RAMON = 0;
#else
    for (int i = 0; i < ARRAY_SIZE(NRF_POWER->RAM); i++) {
        NRF_POWER->RAM[i].POWERCLR = (POWER_RAM_POWERCLR_S1RETENTION_Msk |
                                      POWER_RAM_POWERCLR_S0RETENTION_Msk);
    }
#endif

[dylad]

@benpicco I've updated the commit and squashed.

[dylad]

Thanks for the review !