arm: mach-keystone: Use appended original image size in image processing

When an image is signed/encrypted on K2 devices the image may get padded
to meet alignment requirements for the signature/encryption type. The
original size is appended as 4 bytes little-endian to the end of the
final image.

Normally the trailing extra bytes are ignored and so restoring the
exact original size is not important. In the case of initrd the
original size is important as the kernel uses it to look for
additional filesystem data and can do the wrong thing when the
size is not correct.

Read off the original size and report it back from the image post
processing stage.

Signed-off-by: Andrew F. Davis <afd@ti.com>
Reviewed-by: Lokesh Vutla <lokeshvutla@ti.com>