dt-bindings: arm: fsl: Add imx8mm ddr4 evk board

Add entry for i.MX8MM DDR4 EVK board and update
the comments for imx8mm-evk as it is populated
with LPDDR4.

Signed-off-by: Jacky Bai <ping.bai@nxp.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>