drm/radeon: Expose vddc through hwmon

Create hwmon attribute for vddc, that uses previously declared get_current_vddc() callback if there's an implementation available.

Also hides vddc, if there is no implementation for the current chipset (as per Alexander Deucher's suggestion).

Signed-off-by: Sandeep Raghuraman <sandy.8925@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>