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>

Adds doc/README.ti-secure file to explain in generic terms
how boot images need to be created for secure devices from
Texas Instruments.

Specific details for creating secure boot images for the
AM43xx, DRA7xx and AM57xx secure devices from Texas
Instruments are also provided in the README file.

Secure devices require a security development package (SECDEV)
package that can be downloaded from:

	http://www.ti.com/mysecuresoftware

Login is required and access is granted under appropriate NDA
and export control restrictions.

Signed-off-by: Madan Srinivas <madans@ti.com>
Signed-off-by: Daniel Allred <d-allred@ti.com>

Reviewed-by: Lokesh Vutla <lokeshvutla@ti.com>
Reviewed-by: Andreas Dannenberg <dannenberg@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>