MLK-20553-3 doc: imx: ahab: Add Secure Boot documentation for i.MX8 and i.MX8x SPL targets

The current U-Boot implementation includes SPL targets for i.MX8QM and
i.MXQXP MEK boards:

- imx8qxp_mek_spl_defconfig
- imx8qxp_mek_spl_fspi_defconfig
- imx8qm_mek_spl_defconfig
- imx8qm_mek_spl_fspi_defconfig

The U-Boot proper and ATF are included in an additional container being
necessary a different procedure for signing the flash.bin image.

Add a step-by-step guide covering the signing procedure.
Add a CSF example for the 3rd container.

Signed-off-by: Breno Lima <breno.lima@nxp.com>
Reviewed-by: Frank Zhang <frank.zhang@nxp.com>
Reviewed-by: Marius Grigoras <marius.grigoras@nxp.com>
Reviewed-by: Utkarsh Gupta <utkarsh.gupta@nxp.com>
(cherry picked from commit 04505024d38eebbb5f39133b502c8e450ca40215)
2 files changed