sh4: cache: correct flush_cache() to writeback and invalidate

sh4: cache: correct flush_cache() to writeback and invalidate

In common usecases flush_cache() assumes both cache invalidation and
write-back to memory, instead of doing cache invalidation only with
the wrapped 'ocbi' instruction pin flush_cache() to cache invalidation
with memory write-back done by 'ocbp'.

Signed-off-by: Vladimir Zapolskiy <vz@mleia.com>
Reviewed-by: Simon Glass <sjg@chromium.org>