MIPS: Remove mips_cache_lock() feature

As requested in commit e1390801 ([MIPS]
Request for the 'mips_cache_lock()' removal), such feature is no longer
needed for current MIPS implementation of U-Boot, and no one in the tree
uses it for years.

Signed-off-by: Shinya Kuribayashi <skuribay@pobox.com>