drm/amdgpu/discovery: add SMUIO_13_0_8 func support

Add SMUIO funcs for SMUIO_13_0_8.

Signed-off-by: Chengming Gui <Jack.Gui@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>