MIPS: cache: remove config option CONFIG_SYS_MIPS_CACHE_MODE

Caches should be configured to mode CONF_CM_CACHABLE_NONCOHERENT
(or CONF_CM_CACHABLE_COW when a CM is available). There is no
need to make this configurable.

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