drm/amdgpu: add atom interfaces for CGS

This implements the interface for atombios command
and data table access.

Reviewed-by: Jammy Zhou <Jammy.Zhou@amd.com>
Signed-off-by: Chunming Zhou <David1.Zhou@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>