pwm: atmel: Rework tracking updates pending in hardware

This improves the driver's behavior in several ways:

 - The lock is held for shorter periods and so a channel that is currently
   waited for doesn't block disabling another channel.

 - It's easier to understand because the procedure is split into more
   semantic units and documentation is improved

 - A channel is only set to pending when such an event is actually
   scheduled in hardware (by writing the CUPD register).

 - Also wait in .get_state() to report the last configured state instead
   of (maybe) the previous one. This fixes the read back duty cycle and so
   prevents a warning being emitted when PWM_DEBUG is on.

Tested on an AriettaG25.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>