boards: h3ulcb: Add support of pwm0

Declare pw0 channel0 that can be used on H3ULCB (test pin CP8).
Add support status to H3ULCB board documentation.
PWM0 is disabled by default as its usage could consume uneeded
power.
Only enable it into the test API with overlay.

Signed-off-by: Pierre Marzin <pierre.marzin@iot.bzh>