dt-bindings: memory: Add bindings for imx8m ddr controller

Add devicetree bindings for the i.MX DDR Controller on imx8m series
chips. It supports dynamic frequency switching between multiple data
rates and this is exposed to Linux via the devfreq subsystem.

Signed-off-by: Leonard Crestez <leonard.crestez@nxp.com>
Acked-by: Chanwoo Choi <cw00.choi@samsung.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>