aw_ccung: Support clock factors where factor=0, factor is effectively 1

This happens in two cases for a20 clocks:

pll_core for 'n' factor:
factor=0, val=1
factor=n, val=n

ahb divisor:
factor=0,val=/2
factor=n,val=/2^n

Reviewed by: manu
Differential Revision: https://reviews.freebsd.org/D15806