drm/amdgpu: Read discovery info from system memory

On certain ASICs, discovery info is available at reserved region in system
memory. The location is available through ACPI interface. Add API to read
discovery info from there.

Signed-off-by: Lijo Lazar <lijo.lazar@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>