dts: bindings: clock: add fixed-factor-clock binding

Allows for adding in fixed factor clock that are derived
from a parent clock.

Based on:
https://git.kernel.org/pub/scm/linux/kernel/git/devicetree/devicetree-rebasing.git/tree/Bindings/clock/fixed-factor-clock.yaml



Signed-off-by: Mikkel Jakobsen <mikkel.aunsbjerg@prevas.dk>