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 <bagasdotme@gmail.com>
Signed-off-by: Bagas Sanjaya <bagasdotme@gmail.com>
Signed-off-by: Vadim Fedorenko <vadim.fedorenko@linux.dev>
Signed-off-by: Arkadiusz Kubalewski <arkadiusz.kubalewski@intel.com>
Signed-off-by: Jiri Pirko <jiri@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/driver-api/index.rst

@@ -114,6 +114,7 @@ available subsections can be seen below.
    zorro
    hte/index
    wmi
+   dpll
 
 .. only::  subproject and html