arm: mach-keystone: Implements FIT post-processing call for keystone SoCs

arm: mach-keystone: Implements FIT post-processing call for keystone SoCs

This commit implements the board_fit_image_post_process() function for
the keystone architecture. This function calls into the secure boot
monitor for secure authentication/decryption of the image. All needed
work is handled by the boot monitor and, depending on the keystone
platform, the security functions may be offloaded to other secure
processing elements in the SoC.

The boot monitor acts as the gateway to these secure functions and the
boot monitor for secure devices is available as part of the SECDEV
package for KS2. For more details refer doc/README.ti-secure

Signed-off-by: Vitaly Andrianov <vitalya@ti.com>
Signed-off-by: Madan Srinivas <madans@ti.com>
Signed-off-by: Andrew F. Davis <afd@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Lokesh Vutla <lokeshvutla@ti.com>