drm/amd/powerplay: add vega20 pptable function file

This patch adds the vega20_ppt.c to support ATOM_Vega20_POWERPLAYTABLE format
for vega20 on smu11. It will be used to implement to asic specific pptable
helpers.

Signed-off-by: Huang Rui <ray.huang@amd.com>
Reviewed-by: Likun Gao <Likun.Gao@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>