MIPS: allow systems to skip loads during cache init

Current MIPS systems do not require that loads be performed to force the
parity of cache lines, a simple invalidate by clearing the tag for each
line will suffice. Thus this patch makes the loads & subsequent second
invalidation conditional upon the CONFIG_SYS_MIPS_CACHE_INIT_RAM_LOAD
option, and defines that for existing mips32 targets. Exceptions are
malta where this is known to be unnecessary, and qemu-mips where caches
are not implemented.

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