[MIPS] Fix R4K cache macro names

Several machines have the R4K cache macro name spelled incorrectly.  Namely,
they have cpu_has_4kcache defined instead of cpu_has_4k_cache.

Signed-off-by: Joshua Kinard <kumba@gentoo.org>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>