sh4: cache: correct dcache flush to invalidate with write-back

sh4: cache: correct dcache flush to invalidate with write-back

In common usecases flush_cache() assumes both cache invalidation and
write-back to memory, thus in flush_dcache_range() implementation
change SH4 cache write-back only instruction 'ocbwb' with cache purge
instruction 'ocbp', according to the User's Manual there should be no
performance penalty for that.

Note that under circumstances only cache invalidation is expected from
flush_cache() call, in these occasional cases the current version of
flush_cache() works, which is a wrapper over invalidate_dcache_range()
at the moment, this will be fixed in the following change.

Signed-off-by: Vladimir Zapolskiy <vz@mleia.com>