ARM: dts: ls1021a: Add of_graph dt nodes to describe the panel

add of_graph dt nodes to describe the panel, and removed
"fsl,panel" property

Signed-off-by: Meng Yi <meng.yi@nxp.com>
Acked-by: Stefan Agner <stefan@agner.ch>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>