doc: Updates info on using keystone secure devices, from TI

Add a section describing the secure boot image used on keystone secure devices.

This patch applies on top of the patch
doc: Update info on using AM33xx secure devices from TI submitted by Andrew Davis

---

Changes in v3: None
Changes in v2:
- Updates the secure keystone image name to u-boot_HS_MLO in README.ti-secure to match with the changes made to config.mk in this series version.

  doc/README.ti-secure | 20 ++++++++++++++++++++
  1 file changed, 20 insertions(+)

index 9b0fbf9..84c7206 100644
@@ -133,6 +133,26 @@ Booting of U-Boot SPL
  	u-boot-spl_HS_X-LOADER - boot image for all other flash memories
  		including QSPI and NOR flash
 
+        Invoking the script for Keystone Secure Devices
+        =============================================
+
+        create-boot-image.sh \
+                <UNUSED> <INPUT_FILE> <OUTPUT_FILE> <UNUSED>
+
+        <UNUSED> is currently ignored and reserved for future use.
+
+        <INPUT_FILE> is the full path and filename of the public world boot
+        loader binary file (only u-boot.bin is currently supported on
+	keystone devices, u-boot-spl.bin is not currently supported).
+
+        <OUTPUT_FILE> is the full path and filename of the final secure
+        image. The output binary images should be used in place of the 

standard
+ non-secure binary images (see the platform-specific user's
guides and
+ releases notes for how the non-secure images are typically used)
+ u-boot_HS_MLO - signed and encrypted boot image that can
+ be used to boot from all media. Secure boot from SPI NOR
+ flash is not currently supported.
+
Booting of Primary U-Boot (u-boot.img)
======================================

2.7.4