drm/amd/pm: sysfs attrs to read ss powershare (v6)

add sysfs attrs to read smartshift APU and DGPU power share.
document the sysfs device attributes.

V2: change variable/macro name for stapm power limit (Lijo)
V3: files to be exposed as sysfs device attributes (Alex)
V4: check ret value of sysfs create and remove only if created.
V5: add ss attrs in amdgpu_device_attrs and use attr_update (Lijo)
V6: all checks for ss support to be in if else if statements. (Lijo)

Signed-off-by: Sathishkumar S <sathishkumar.sundararaju@amd.com>
Reviewed-by: Lijo Lazar <lijo.lazar@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>