sh: Zero out aliases counter when using SH-X3 hardware assistance.

This zeroes out the number of cache aliases in the cache info descriptors
when hardware alias avoidance is enabled. This cuts down on the amount of
flushing taken care of by common code, and also permits coherency control
to be disabled for the single CPU and 4k page size case.

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