TIMG: Not all timer clock sources are set correctly

[MabezDev]

The important line here is the

Figure12-2 is a diagram of timer Tx in a timer group

Meaning that each timer inside a timer group can configure its clock source. @burrbull pointed out in #1364 (comment) that we currently aren't doing that.

The reason we haven't seen any issues stemming from this oversight, is because the reset value is 0, which means by default all timers will use the APB clock.