MIPS: R5000: Fix TLB hazard handling.

R5000 and the Nevada CPUs (RM5230, RM5231, RM5260, RM5261, RM5270 and
RM5271) are basically the same CPU core and all are documented to require
two instructions separating a write to c0_pagemask, c0_entryhi, c0_entrylo0,
c0_entrylo1 or c0_index.

So far we were only providing on cycle before / after a TLBR/TLBWI
for R5000 but 3 cycles before and 1 cycles after for the Nevadas.

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