MIPS: asm: r4kcache: Build flushing code for instruction cache

Build code to invalidate an address range in the  instruction cache
using the Hit Invalidate cache operation.

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