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.

v9->v10:
- fix muxed get-pin command respond example
- fix typos
- describe DPLL_MODE_FREERUN
- describe DPLL_A_PIN_PHASE_ADJUST
- describe DPLL_A_PHASE_SHIFT

v8->v9:
- fix docs build warnings
- separate netlink command/attribute list
- replace enum description with uapi header
- add brief explanation what is a DPLL
- fix EOPNOTSUPP typo
- fix typo .state_get -> .state_on_dpll_get

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>

Author: Vadim Fedorenko