drm/amd/amdgpu: add SI defines/registers

Add missing gca MMIO registers and defines necessary for the
next patch which re-works a lot of gfx v6 to use the new SI
headers.

Signed-off-by: Tom St Denis <tom.stdenis@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>