change(backlight): Keep PWM stable when changing PCLK

Timer frequency is now held close to a value which can be achieved both
with a 48 MHz PCLK (96 MHz system clock) and a 7.5 MHz PCLK (15 MHz
system clock).

If even lower PCLKs should be supported, the frequency of the timer has
to be changed as well.
2 jobs for !478 with schneider/backlight-scaling in 30 seconds (queued for 7 seconds)
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