doc: imx: habv4: Add Secure Boot guide for i.MX8M SPL targets

doc: imx: habv4: Add Secure Boot guide for i.MX8M SPL targets

Add HABv4 documentation extension for SPL targets covering the
following topics:

- How to sign an securely boot an flash.bin container image.
- How to extend the root of trust for additional boot images.
- Add SPL and fitImage CSF examples.
- Add signature generation script example.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Breno Lima <breno.lima@nxp.com>
Cc: Fabio Estevam <festevam@denx.de>
Cc: Heiko Schocher <hs@denx.de>
Cc: Peng Fan <peng.fan@nxp.com>
Cc: Stefano Babic <sbabic@denx.de>
Cc: Utkarsh Gupta <utkarsh.gupta@nxp.com>
Cc: Ye Li <ye.li@nxp.com>