Add PWM module

[kalkyl]

Added a HAL module for the PWM peripheral

LED demo of some different modes and features (nRF52840-DK):
https://github.com/kalkyl/nrf-hal/blob/pwm/examples/pwm-demo/src/main.rs

[Mirabellensaft]

Hi! I have looked over your PR, the pwm.rs looks good to me so far. Looking at the demo, it would be really helpful to have comments that explain what the different parts of the demo are about.

[kalkyl]

Thanks, good idea!

[huntc]

Thanks for this great contribution @kalkyl.

Being a first-time user of PWM, I did find the example introducing many new concepts. I'm sure the example will be really useful to me over time, but I was wondering if you knew of "blinky-style" examples using Pwm and an RGB LED? I'm presently writing a BSP for the Thingy:91 and struggling a little: https://users.rust-lang.org/t/guidance-for-using-pwm-on-the-thingy-91/59516. I'd be happy to contribute an additional example here though if I can get mine to work.