drm/amdgpu: add nbio7 support

NBIO handles misc bus io functions on the chip.  This
helper lib has the apppropriate functions for NBIO 7.0.

Signed-off-by: Chunming Zhou <David1.Zhou@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>