MIPS: asm: r4kcache: Add MIPS R6 cache unroll functions

MIPS R6 changed the 'cache' instruction opcode and reduced the
offset field to 8 bits. This means we now have to adjust the
base register every 256 bytes and as a result of which we can
no longer use the previous cache functions.

Signed-off-by: Markos Chandras <markos.chandras@imgtec.com>