MLK-20270-1 doc: imx: habv4: Add Secure Boot documentation for i.MX8M and i.MX8MM devices

Add HABv4 documentation for i.MX8M and i.MX8MM targets covering the
following topics:

- How to sign an securely boot an flash.bin image.
- How to extend the root of trust for additional boot images.
- Add 2 CSF examples.

Reviewed-by: Utkarsh Gupta <utkarsh.gupta@nxp.com>
Signed-off-by: Breno Lima <breno.lima@nxp.com>
(cherry picked from commit cc63be298a3e5f44e417f4098c124715917d09e1)
3 files changed