drm/amdgpu: add RREG64/WREG64(_PCIE) operations

add 64 bits register access functions

v2: implement 64 bit functions in low level

Signed-off-by: Tao Zhou <tao.zhou1@amd.com>
Reviewed-by: Dennis Li <dennis.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>