This commit refactors machine.PWM and creates extmod/machine_pwm.c. The
esp8266, esp32 and rp2 ports all use this and provide implementations of
the required PWM functionality. This helps to reduce code duplication and
keep the same Python API across ports.
This commit does not make any functional changes.
Signed-off-by: Damien George <damien@micropython.org>
Damien George