Each of the six available timers now handles two consecutive PWM output
channels. This also implements support for changing the group PWM
frequency in a similar manner to the ChibiOS HAL.
Enabling/disabling the timer would apply the setting to whole groups of
channels. Fix to poke the comparator so that the setting only applies to
the particular channel.
Conveniently, though not necessarily intentionally, this avoids
truncating the output pulse and causing unexpected reactions from
servos. This also preserves the old behavior.
