[WHY]
Power down VPG and AFMT blocks when not in use

[HOW]
Create afmt31 and vpg31 structs and add necessary fields to reg list
Reviewed-by: Eric Yang <eric.yang2@amd.com>
Acked-by: Mikita Lipski <mikita.lipski@amd.com>
Signed-off-by: Michael Strauss <michael.strauss@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>