MIPS: unify CPU code in arch/mips/cpu/

Unify and move code in arch/mips/cpu/mips[32|64]/ to arch/mips/cpu/.
The CPU specific config.mk files need to remain until
CONFIG_STANDALONE_LOAD_ADDR is converted to a global Kconfig symbol.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Signed-off-by: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>