drm/amdgpu: add the files of NBIO v7.7 block

This patch is to add the files of NBIO v7.7 block.
NBIO (New Bus IO) is the block which handles
the GPU interface to the PCIe bus.

v2: squash in register name fix (Xiaojian)

Signed-off-by: Xiaojian Du <Xiaojian.Du@amd.com>
Reviewed-by: Huang Rui <ray.huang@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>