Add xgmi_plpd_policy sysfs node for users to check and select xgmi
per-link power down policy:
  - arg 0: disallow plpd
  - arg 1: default policy
  - arg 2: optimized policy

v2: split from smu v13.0.6 code and miscellaneous updates
v3: add usage comments around set/get functions
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>