ARM: dts: add new compatible string for i.MX6QP mmdc

MMDC has a slightly different programming model between imx6q and imx6qp
in terms of perf support, it's exactly same for suspend support, so we
have fsl,imx6q-mmdc here to save patching suspend driver with the new
compatible.

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