[MIPS] Fix and cleanup the MIPS part of the (ab)use of CLOCK_TICK_RATE.

This is the clock rate of the i8253 PIT.  A MIPS system may not have
a PIT by the symbol is used all over the kernel including some APIs.
So keeping it defined to the number for the PIT is the only sane thing
for now.

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>