doc: SPL: Add README for secure boot support

Adds information regarding SPL handling validation process of main u-boot
image on power/mpc85xx and arm/layerscape platforms.

Signed-off-by: Sumit Garg <sumit.garg@nxp.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: York Sun <york.sun@nxp.com>