dpll: documentation on DPLL subsystem interface

Add documentation explaining common netlink interface to configure DPLL
devices and monitoring events. Common way to implement DPLL device in
a driver is also covered.

Co-developed-by: Bagas Sanjaya <[email protected]>
Signed-off-by: Bagas Sanjaya <[email protected]>
Signed-off-by: Vadim Fedorenko <[email protected]>
Signed-off-by: Arkadiusz Kubalewski <[email protected]>
Signed-off-by: Jiri Pirko <[email protected]>
Signed-off-by: David S. Miller <[email protected]>

Author: Vadim Fedorenko