gma500: Add Oaktrail support

Oaktrail (GMA600) is found on some tablet/slate PC type systems. It's a bit
different to the GMA500 but similar enough it makes sense to plug it into
the same driver.

Signed-off-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>