doc: Update info on using secure devices from TI

Adds information regarding SPL handling the loading and processing of
secured u-boot images as part of the second stage boot the SPL does.
Introduces the description of a new interface script in the TI SECDEV
Package which handles the creation and prep of secured binary images.

Signed-off-by: Daniel Allred <d-allred@ti.com>
Signed-off-by: Andreas Dannenberg <dannenberg@ti.com>
Reviewed-by: Simon Glass <sjg@chromium.org>