cpu/efm32/pm: fix blocking EM1

[jue89]

Contribution description

This PR enable users to block EFM32_PM_MODE_EM1 if required.

Testing procedure

Programming tests/periph_pm to an EFM32-based board:

2022-10-19 17:56:56,151 # main(): This is RIOT! (Version: 2023.01-devel-93-g345af-feature/efm32-series2)
2022-10-19 17:56:56,157 # This application allows you to test the CPU power management.
2022-10-19 17:56:56,214 # The available power modes are 0 - 2. Lower-numbered power modes
2022-10-19 17:56:56,217 # save more power, but may require an event/interrupt to wake up
2022-10-19 17:56:56,218 # the CPU. Reset the CPU if needed.
2022-10-19 17:56:56,218 # mode 0 blockers: 0 
2022-10-19 17:56:56,218 # mode 1 blockers: 1 
2022-10-19 17:56:56,218 # mode 2 blockers: 0 
2022-10-19 17:56:56,219 # Lowest allowed mode: 2
2022-10-19 17:56:56,219 # [pm] enter EFM32_PM_MODE_EM1
> pm block 2
2022-10-19 17:57:28,977 # pm block 2
2022-10-19 17:57:28,978 # Blocking power mode 2.
> pm show
2022-10-19 17:57:42,618 # pm show
2022-10-19 17:57:42,619 # mode 0 blockers: 0 
2022-10-19 17:57:42,620 # mode 1 blockers: 1 
2022-10-19 17:57:42,620 # mode 2 blockers: 1 
2022-10-19 17:57:42,621 # Lowest allowed mode: 3
> pm unblock 2
2022-10-19 17:57:58,481 # pm unblock 2
2022-10-19 17:57:58,482 # Unblocking power mode 2.
2022-10-19 17:57:58,485 # [pm] enter EFM32_PM_MODE_EM1

Issues/PRs references

#13475 yielded #17895, which already prepared the EFM32 CPUs to take the IDLE pm into account.
#17883 implements the same logic for the samd5x cpu family.

[riot-ci]

Murdock results

✔️ PASSED

67b3800 cpu/efm32/pm: add debug output

Success	Failures	Total	Runtime
1991	0	1991	06m:24s

Artifacts

• Documentation preview

This only reflects a subset of all builds from https://ci-prod.riot-os.org. Please refer to https://ci.riot-os.org for a complete build for now.

[jue89]

Thank you :)