MIPS: Flush data cache upon relocation

This patch now adds a flush to the data cache upon relocation. The
current implementation is missing this. Only a comment states that it
should be done. So let's really do it now.

Signed-off-by: Stefan Roese <sr@denx.de>
Signed-off-by: Shinya Kuribayashi <skuribay@ruby.dti.ne.jp>