MIPS: avoid .set ISA for cache operations

As a step towards unifying the cache maintenance code for mips32 &
mips64 CPUs, stop using ".set <ISA>" directives in the more developed
mips32 version of the code. Instead, when present make use of the GCC
builtin for emitting a cache instruction. When not present, simply don't
bother with the .set directives since U-boot always builds with
-march=mips32 or higher anyway.

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