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 <[email protected]>
Cc: Breno Lima <[email protected]>
Cc: Fabio Estevam <[email protected]>
Cc: Heiko Schocher <[email protected]>
Cc: Peng Fan <[email protected]>
Cc: Stefano Babic <[email protected]>
Cc: Utkarsh Gupta <[email protected]>
Cc: Ye Li <[email protected]>