sparc64: add per-cpu mm of secondary contexts

The new wrap is going to use information from this array to figure out
mm's that currently have valid secondary contexts setup.

Signed-off-by: Pavel Tatashin <pasha.tatashin@oracle.com>
Reviewed-by: Bob Picco <bob.picco@oracle.com>
Reviewed-by: Steven Sistare <steven.sistare@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>