MIPS: Abstract cache op loops with a macro

The various cache maintenance routines perform a number of loops over
cache lines. Rather than duplicate the code for performing such loops,
abstract it out into a new cache_loop macro which performs an arbitrary
number of cache ops on a range of addresses. This reduces duplication in
the existing L1 cache maintenance code & will allow for not adding
further duplication when introducing L2 cache support.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>