ARM: dts: imx51: Fix inconsistent display port names

Contrary to later i.MX SoCs, the parallel display interface pad groups on
i.MX51 are called DISP1 and DISP2 in the Reference Manual, not DISP0 and
DISP1.

Fix this inconsistence by changing the DISP names in the i.mx51 dts.

Signed-off-by: Marco Franchi <marco.franchi@nxp.com>
Acked-by: Philipp Zabel <p.zabel@pengutronix.de>
Reviewed-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>