Add support for PWM module

[haata]

The PWM module on atsam4 is a bit confusing as it has High and Low output pins. I believe these are intentioned for motor controllers as they are inverted pairs.

Effectively there are 4 PWM channels for both atsam4e and atsam4s.

Clock configuration is also a bit confusing.
There are 4 channels, but only 2 fully customizable clocks (see CLKA and CLKB). These clocks can be used on multiple channels (as well as a bunch of peripheral clock divisions). See 39.6.1 (either atsam4s or atsam4e) for more details.

https://docs.rs/embedded-hal/0.2.4/embedded_hal/trait.Pwm.html
https://docs.rs/embedded-hal/0.2.4/embedded_hal/trait.PwmPin.html