cpu/stm32wl: Enable rtc support

[FlapKap]

Contribution description

This PR adds support for the real-time clock on the stm32wl series devices. Based on information from https://www.st.com/resource/en/reference_manual/dm00451556-stm32wl5x-advanced-armbased-32bit-mcus-with-subghz-radio-solution-stmicroelectronics.pdf

Testing procedure

Issues/PRs references

[krzysztof-cabaj]

Nice PR.

One remark look at commit conventions.

[riot-ci]

Murdock results

:heavy_check_mark: PASSED

54ad88c90e01aa6d534cfd57cb91186bc0badc70 initial

Success	Failures	Total	Runtime
10027	0	10027	11m:02s

Artifacts

• Documentation preview

[FlapKap]

Nice PR.

One remark look at commit conventions.

Thanks Krzystof. I will fix it before final review. Currently I haven't gotten alarms to work yet so this PR is mainly to gather advice on that