This commit allows using all the available PWM timers (up to 8) and
channels (up to 16), without affecting the PWM API.
If a new frequency is set, first it checks if another timer is using the
same frequency. If yes, then it uses this timer, otherwise, it creates a
new one. If all timers are used, the user should set an already used
frequency, or de-init a channel.
This work is based on #6276 and #3608.
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