[PARISC] Use CONFIG_HZ to determine interval timer rate (aka clock ticks)

This isn't likely to be causing problems for other bits of
kernel code. I can't find any other user of CONFIG_HZ outside
of arch specific code.

Signed-off-by: Grant Grundler <grundler@parisc-linux.org>
Signed-off-by: Kyle McMartin <kyle@parisc-linux.org>