blink_led sample does not work on most of the nRF boards

[anangl]

The issue was originally reported on slack (#general):

Aaron Tsui 9:15 AM, March 22nd
BTW, As the PWM_CLK lowest is 125khz(16M/128) and COUNTERTOP max is 0x7fff on nrf52, my understanding the nrf52 HW PWM longest period is about 262ms, cannot reach period as 1000ms. right?
@gaute this will cause failure on sample/blink_led, which need a 1000ms period.

Indeed, when trying to run this sample on any board with an nRF52 series SoC, which by default uses the HW PWM driver, one hits the following error:

PWM demo app-blink LED
pwm pin set fails
[00:00:00.008,544] <err> pwm_nrfx: Prescaler for period_cycles 16000000 not found.

[00:00:00.008,544] <err> pwm_nrfx: Incompatible period 16000000

Same happens for boards with nRF91 series SoCs.

For nRF52 the solution could be to use the nRF SW PWM driver, for nRF91 the driver is unavailable.

[carlescufi]

@pabigot could you please check if this is still an issue?

[pabigot]

@carlescufi I believe PR #15287 needs to be rebased and merged first. Right now the build fails on nRF52 because there's no devicetree entry for the required hardware component.

Also the Nordic board definitions seem to be missing the #pwm-cells property which is required to provide the alias to the hardware node (at least using the same approach as in nxp and other boards).

So yes, this still needs work.