sh: NO_CONTEXT ASID optimizations for SH-4 cache flush.

This optimizes for the cases when a CPU does not yet have a valid ASID
context associated with it, as in this case there is no work for any of
flush_cache_mm()/flush_cache_page()/flush_cache_range() to do. Based on
the the MIPS implementation.

Signed-off-by: Paul Mundt <lethal@linux-sh.org>