ARM: dts: sirf: add missed graphics2d iobg in atlas6 dts

there is a bus bridge for graphics 2D module lost in current dts, this patch takes it
back.

Signed-off-by: Jiansong Chen <jiansong.chen@csr.com>
Signed-off-by: Barry Song <Baohua.Song@csr.com>
Signed-off-by: Olof Johansson <olof@lixom.net>