sparc64: use prom interface to get %stick frequency

sparc64: use prom interface to get %stick frequency

We initialize time early, we must use prom interface instead of open
firmware driver, which is not yet initialized.

Also, use prom_getintdefault() instead of prom_getint() to be compatible
with the code before early boot timestamps project.

Signed-off-by: Pavel Tatashin <pasha.tatashin@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Reviewed-by: Steven Sistare <steven.sistare@oracle.com>
Orabug: 24401250
Orabug: 25637776

(cherry picked from commit fca4afe400cb68fe5a7f0a97fb1ba5cfdcb81675)
Signed-off-by: Pavel Tatashin <pasha.tatashin@oracle.com>
Signed-off-by: Allen Pais <allen.pais@oracle.com>