drm/amd/powerplay: add a common pp endian header

To replace the duplicated versions of this in all asic
variants.

Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>
Reviewed-by: Ken Wang <Qingqing.Wang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>