amdgpu/pm: add kernel documentation for smu_get_power_limit

 added doc tag "amdgpu_pp_power" with description
 added tags for enums  pp_power_limit_level, pp_power_type
 added tag for function smu_get_power_limit

Test:
* Temporary insertion into Documentation/gpu/amdgpu.rst
------------START------------
Power Limit
-----------
.. kernel-doc:: drivers/gpu/drm/amd/include/kgd_pp_interface.h
   :doc: amdgpu_pp_power

.. kernel-doc:: drivers/gpu/drm/amd/include/kgd_pp_interface.h
   :identifiers: pp_power_limit_level

.. kernel-doc:: drivers/gpu/drm/amd/include/kgd_pp_interface.h
   :identifiers: pp_power_type

.. kernel-doc:: drivers/gpu/drm/amd/pm/swsmu/amdgpu_smu.c
   :identifiers: smu_get_power_limit
-------------END-------------

Signed-off-by: Darren Powell <darren.powell@amd.com>
Reviewed-by: Lijo Lazar <lijo.lazar@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>