MIPS: BMIPS: Add special cache handling in c-r4k.c
BMIPS435x and BMIPS438x have a single shared L1 D$ and load/store unit,
so it isn't necessary to raise IPIs to keep both CPUs coherent.
BMIPS5000 has VIPT L1 caches that handle aliases in hardware, and its I$
fills from D$. But a special sequence with 2 SYNCs and 32 NOPs is needed
to ensure coherency.