arm64: dts: freescale: Add i.MX8MN dtsi support

The i.MX8M Nano Media Applications Processor is a new SoC of the i.MX8M
family, it is a 14nm FinFET product of the growing mScale family targeting
the consumer market. It is built in Samsung 14LPP to achieve both high
performance and low power consumption and relies on a powerful fully
coherent core complex based on a quad core ARM Cortex-A53 cluster,
Cortex-M7 low-power coprocessor and graphics accelerator.

This patch adds the basic dtsi support for i.MX8MN.
Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>