boards/nrf52840dongle: Configure PWM to drive the LEDs

[chrysn]

Contribution description

The nRF52840-Donle board has hardware that lends itself to PWM driving, but did not configure a default PWM setting.

This adds a single PWM peripheral that is configured for the LEDs LD1, LD2#red, LD2#green and LD2#blue.

Testing procedure

$ make BOARD=nrf52840dongle -C tests/periph_pwm all flash term
[...]
> osci

and watch the green LED and the roughly-white-appearing RGB LED fade in unison.

The LEDs are driven active-low, which is of no concern to the PWM module but will help understand the values when you test that the documented assignment ("is set to drive the LEDs LD1 and the channels LD2 red, green and blue in the four channels of PWM_DEV(0)") matches:

> init 0 0 1000 255
> set 0 0 255 # turn off LD1
> set 0 1 255 # turn off LD2#red, leaving it turquise
> set 0 2 250 # turn off almost all of LD2#green, leaving it blue
> set 0 3 255 # turn off LD2#blue, leaving the faint green from before

Issues/PRs references

This is done a) because it's a peripheral that can reasonably expected to be wired, and b) as a test case for #15106.

[benpicco]

works like a charm