cpu/rpx0xx: add initial pwm support

[dylad]

Contribution description

This PR adds an initial PWM peripheral driver for RP2040-based board.
PWM IP block is feed by the system clock (fixed) so it is not very flexible but it works as intended.
So far I've only add the LED pin as PWM as it is convenient to check with the osci command from tests/periph/pwm
However, I've also tested locally other pins to check the overall logic.

Testing procedure

Play with tests/periph/pwm on rpi-pico for instance.

Issues/PRs references

None.

[dylad]

With a logic analyzer attached and the following commands:

2024-03-15 16:27:48,435 # init 1 0 20000 100
2024-03-15 16:27:48,437 # The pwm frequency is set to 20161
set 1 0 50
2024-03-15 16:27:55,113 # set 1 0 50
> set 1 0 25
2024-03-15 16:29:35,330 # set 1 0 25
> set 1 0 95
2024-03-15 16:30:33,831 # set 1 0 95

Here are the results:

[kfessel]

reads good, test results provided

please squash your fixups

[dylad]

Rebased and squashed.

[riot-ci]

Murdock results

✔️ PASSED

8b84fc5 boards/rpi-pico: add PWM support in Kconfig

Success	Failures	Total	Runtime
10009	0	10009	07m:19s

Artifacts

[kfessel]

trusting @dylad tests

[dylad]

Thanks for the review @kfessel !