ARM: dts: imx53: Fix LDB OF graph warning

Single child nodes in OF graph don't need an address and now dtc will
warn about this:

Warning (graph_child_address): /soc/aips@50000000/ldb@53fa8008/lvds-channel@0: graph node has single child node 'port@0', #address-cells/#size-cells are not necessary

Since the LDB should always have an output port, fix the warning by
adding the output port, 2, to the DT.

Cc: Sascha Hauer <s.hauer@pengutronix.de>
Cc: Pengutronix Kernel Team <kernel@pengutronix.de>
Cc: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Rob Herring <robh@kernel.org>
Reviewed-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>