MIPS: Ensure Config.K0=2 applies before any memory accesses

MIPS: Ensure Config.K0=2 applies before any memory accesses

During boot we set Config.K0=2 (uncached) such that any accesses to the
kseg0 memory region are performed uncached before the caches are
initialised. This write to the Config register introduces an execution
hazard between it & any following memory accesses (such as the load of
_gp), which we need to clear in order to ensure those memory accesses
are actually performed uncached. Clear this execution hazard with the
insertion of an ehb execution hazard barrier instruction.

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