[MIPS] Implement flush_cache()

We do Hit_Writeback_Inv_D and Hit_Invalidate_I. You might think that you
don't need to do Hit_Invalidate_I, but flush_cache() needs it since this
function is used not only in U-Boot specfic programs but also at loading
target binaries.

Signed-off-by: Shinya Kuribayashi <skuribay@ruby.dti.ne.jp>