dt-bindings: memory-controllers: freescale: add MMDC binding doc

Freescale MMDC (Multi Mode DDR Controller) driver is supported
since i.MX6Q, but not yet documented, this patch adds binding
doc for MMDC module driver.

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