clock: Add dynamic frequency control

[eisental]

In addition to fixed clock frequency add the option for a frequency range.
Any number of additional inputs can set a specific frequency in this range.

Example:
clock
100..200ms

with 2 freq inputs.
when freq inputs value is 0 the clock works at 100ms per tick
when freq inputs value is 1 the clock works at 133ms
when value is 2 the clock works at 166ms
when value is 3 the clock works at 200ms