Add the interface to change xgmi per-link power down policy.

v2: split from sysfs interface code and miscellaneous updates
v3: check against XGMI_PLPD_DEFAULT/XGMI_PLPD_OPTIMIZED and
    pass PPSMC param
Signed-off-by: Le Ma <le.ma@amd.com>
Reviewed-by: Asad Kamal <asad.kamal@amd.com>
Reviewed-by: Lijo Lazar <lijo.lazar@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>