drm/amd/pm: Add xgmi plpd to aldebaran pm_policy

On aldebaran, allow changing xgmi plpd policy through
'pm_policy/xgmi_plpd' sysfs interface.

Signed-off-by: Lijo Lazar <lijo.lazar@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Reviewed-by: Asad Kamal <asad.kamal@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>