drm/amdgpu/include: Add nbio 7.4 header files (v4)

v2: Cleanups (Alex)
v3: More updates (Alex)
v4: more cleanups (Alex)

Signed-off-by: Feifei Xu <Feifei.Xu@amd.com>
Acked-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>