ARM: dts: imx6qdl: Improve mmdc1 node

Add MMDC1 compatible string which is missing, and also set
it to be disabled by default, as most of the platforms ONLY
use single channel MMDC0, if dual MMDC channels are used, it
can be enabled in board dts file.

Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>