drm/amdxcp: add platform device driver for amdxcp

Add platform device driver for amdxcp to support
amdgpu spatial partition.

-v2: fix build warning

Signed-off-by: James Zhu <James.Zhu@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>