drm/amd/include: Add mmhub 9.4 reg offsets and shift-mask

In particular, we need the mmMC_VM_XGMI_LFB_CNTL register, for
determining if xGMI is enabled on VG20. This will be used by DC to
determine the correct spread spectrum adjustment for display and audio
clocks.

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>