sparc64: improve modularity tick options

This patch prepares the code for early boot time stamps by making it more
modular.

- init_tick_ops() to initialize struct sparc64_tick_ops
- new sparc64_tick_ops operation get_frequency() which returns a
  frequency

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>