ARM: AM57xx: Enable post-processing of FIT artifacts loaded by U-Boot

ARM: AM57xx: Enable post-processing of FIT artifacts loaded by U-Boot

Enable the platform-specific post-processing of FIT-extracted blobs such
as Kernel, DTB, and initramfs on TI AM57xx high-security (HS) devices
which will ultimately invoke a ROM-based API call that performs secure
processing such as blob authentication.

Signed-off-by: Andreas Dannenberg <dannenberg@ti.com>
Signed-off-by: Andrew F. Davis <afd@ti.com>
Reviewed-by: Lokesh Vutla <lokeshvutla@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>