drm/amdgpu: support ASPM for some specific ASIC

Support to program ASPM and LTR for Sienna Cichlid and forward ASIC.
Disable ASPM for Sienna Cichlid and forward ASIC by default.

Signed-off-by: Likun Gao <Likun.Gao@amd.com>
Reviewed-by: Kenneth Feng <kenneth.feng@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>