scripts: checkpatch: add Atmel Pwm typedef

Add Atmel Pwm typedef so that when doing `Pwm * const pwm` definition
checkpatch doesn't complain about the position of the '*'. If not added,
we must define `Pwm *const pwm` to make checkpatch happy, but,
clang-format results in `Pwm * const pwm` (correct).

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>