MIPS: cache: reimplement dcache_[status, enable, disable]

MIPS: cache: reimplement dcache_[status, enable, disable]

Those functions are not needed during cache init and can be
implemented in C. Only support the safe disabling of caches when
this is required for booting an OS. Reenabling caches is much
harder to implement if an optional coherency manager must be
supported. As there is no real use-case anyway, dcache_enable
is implemented with an error message.

Signed-off-by: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>