sh: Convert sh64 /proc/asids to debugfs and generic sh.

This converts the sh64 /proc/asids entry to debugfs and enables it for
all SH parts that have debugfs enabled.

On MMU systems this can be used to determine which processes are using
which ASIDs which in turn can be used for finer grained cache tag
analysis.

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