When the ARM global timer initializes, its current rate is declared
to the kernel and maintained forever. Should it's parent clock
change, the driver tries to fix the timer's internal prescaler.
On some machs (i.e. Zynq) the initial prescaler value thus poses
bounds about how much the parent clock is allowed to decrease or
increase wrt the initial clock value.
This affects CPU_FREQ max delta from the initial frequency.