drm/amdgpu: Add mmhub v4_2_0 ip headers v5

Add header files for mmhub v4_2_0 register offsets
and shift masks
v2: Update mmhub v4_2_0 ip headers
v3: Update mmhub v4_2_0 ip headers
v4: Clean up registers (Alex)
v5: Clean up registers (Alex)

Signed-off-by: Hawking Zhang <Hawking.Zhang@amd.com>
Reviewed-by: Likun Gao <Likun.Gao@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>