Add padding at the end of the last entry

This patch adds padding bytes at the end of the last image in the
fip to be able to transfer by DMA the last image.

Change-Id: I8c6f07dee389cb3d1dc919936d9d52841d7e5723
Signed-off-by: Roberto Vargas <roberto.vargas@arm.com>
Signed-off-by: Dimitris Papastamos <dimitris.papastamos@arm.com>
Signed-off-by: David Cunado <david.cunado@arm.com>