MIPS: Add flush_dcache_range() and invalidate_dcache_range()

This patch adds flush_/invalidate_dcache_range() to the MIPS architecture.
Those functions are needed for the upcoming dcache support for the USB
EHCI driver. I chose this API because those cache handling functions are
already present in the PPC architecture.

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