ARM: dts: imx6qp: reference MMDC node by handle and remove duplication

Referencing the node by handle make the QP DT more resilent against
changes of the base DT. Also remove the duplicated reg property, it's
not needed as it the same as in the base DT, just the compatible is
actually different.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>