plat: imx8m: Add the basic support for imx8mm

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 add the basic support for i.MX8MM.

Signed-off-by: Jacky Bai <ping.bai@nxp.com>