docs(imx8m): update for high assurance boot

Add a section into documentation listing the support for High Assurance
Boot (HABv4), note on the DRAM mapping, and reference to the external
documentation.

Signed-off-by: Andrey Zhizhikin <andrey.zhizhikin@leica-geosystems.com>
Change-Id: Iaca97f4ac2595e35de2664a880394519f96eca07