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>