drm/amd/powerplay: Add read_sensor() callback to hwmgr (v3)

Provides standardized interface to read various sensors.
The API is extensible (by adding to the end of the
amd_pp_sensors enumeration list.

Support has been added to Carrizo/smu7

(v2) Squashed the two sensor patches into one.
(v3) Updated to apply to smu7_hwmgr instead

Signed-off-by: Tom St Denis <tom.stdenis@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>