[SPARC64]: Provide mmu statistics via sysfs.

If the system supports hypervisor based statistics, allow them to
be fetched, enabled, and disabled via sysfs.

Enable and disable via the boolean:

/sys/devices/systems/cpu/cpuN/mmustat_enable

Statistic values are provided under:

/sys/devices/systems/cpu/cpuN/mmu_status/

Signed-off-by: David S. Miller <davem@davemloft.net>