MIPS: mm: c-r4k: Set the correct ISA level

The local_r4k_flush_cache_sigtramp function uses the 'cache'
instruction inside an asm block. However, MIPS R6 changed the
opcode for the cache instruction and as a result of which we
need to set the correct ISA level.

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