arm: move fixed factor and rate to generic clock framework

fixed factor and fixed rate clocks are rather generic an can
should be generalised.  This commits makes those part of the
generic clock framework.

lgtm natano@
arm: fix interrupt ack another time

The IAR register also holds the CPUID.  We need to mask only
the bits that we are interested in.
Take the recorded snapshot size into account when reading.

This fixes a bug that triggered a brute-force search for a valid header
when the archive was read successfully and there was sufficient padding
for another tmpfs snapshot header to fit.

ok pedro@