drm/amd/amdgpu: Add debugfs entries for smc/didt/pcie

This adds 3 new files that can be read/written to access
indirect GPU registers.

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>