arm64: dts: imx: Add i.mx8mm dtsi support

The i.MX8M Mini is new SOC of the i.MX8M family. it is
focused on delivering the latest and greatest video and
audio experience combining state-of-the-art media-specific
features with high-performance processing while optimized
for lowest power consumption. The i.MX 8M Mini Media Applications
Processor is  14nm FinFET product of the growing i.MX8M family
targeting the consumer & industrial market. It is built in 14LPP
to achieve both high performance and low power consumption
and relies on a powerful fully coherent core complex based on
a quad Cortex-A53 cluster with video and graphics accelerators

This patch adds the basic dtsi support for i.MX8MM.
Signed-off-by: Jacky Bai <ping.bai@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>