pwm_test: fix some issues
pwm_test: Clean-up and make it work on DT correctly

Clean up and DTify the pwm_test driver into something that's reasonably
useful.
Based on work done by Elias & Jack Mitchel.

Signed-off-by: Pantelis Antoniou <panto@antoniou-consulting.com>
capes: Add PWM test example cape

A simple example of a PWM test based cape.

Signed-off-by: Pantelis Antoniou <panto@antoniou-consulting.com>