radeon: add support for rs600 GPUs

RS600s are an AMD IGP for Intel CPUs, that look like RS690s from
a lot of perspectives but look like r600s from a memory controller
point of view.

Signed-off-by: Alex Deucher <alexdeucher@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
3 files changed