drm/radeon: add r6xx/r7xx microcode

This uses the same microcode system as the current radeon code.

It should be converted to the new microcode loader I suppose,
though really I need a lot more proof of the worth of me maintaining
firmware blobs externally.
Signed-off-by: Alex Deucher <alexdeucher@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>