drm/amdgpu: add gfxhub v2.0 block for navi10 (v4)

gfxhub is the memory controller hub for gfx and sdma.

v1: add place holder and initial basic functions (Ray)
v2: replace the refernce to legacy mc structure with gmc structure
    remove the direct use of gart.table_addr (Hawking)
v3: switch to use amdgpu_gmc_pd_addr (Hawking)
v4: squash in updates (Alex)

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