drm/amd/soc21: enable mmhub and athub power gating

add the pg_flag for athub and mmhub pg.

Signed-off-by: Kenneth Feng <kenneth.feng@amd.com>
Reviewed-by: Jack Gui <Jack.Gui@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>