drm/amdgpu: introduce pcie port read/write entry

This patch adds pcie port read/write entry, because it will be also
used on si dpm part.

Acked-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Huang Rui <ray.huang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>