arch/mips: simplify cache operations

arch/mips: simplify cache operations

Cache operations are simplified by removing assembly
implementation and replacing it with simpler C code.

BUG=chrome-os-partner:31438
TEST=tested on Pistachio bring up board; caches are properly
     invalidated;
BRANCH=none

Change-Id: I0f092660549c368e98c208ae0c991fe6f5a428d7
Signed-off-by: Patrick Georgi <pgeorgi@chromium.org>
Original-Commit-Id: bf99849e75813cba865b15af9e110687816e61e4
Original-Change-Id: I965e7929718424f92f3556369d36a18ef67aa0d0
Original-Signed-off-by: Ionela Voinescu <ionela.voinescu@imgtec.com>
Original-Reviewed-on: https://chromium-review.googlesource.com/250792
Original-Reviewed-by: David Hendricks <dhendrix@chromium.org>
Reviewed-on: http://review.coreboot.org/9820
Tested-by: build bot (Jenkins)
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>