Makefile: Align fit-dtb.blob and u-boot.itb by 64bits

Enabling MULTI_DTB_FIT and DTB_RESELECT can end up with multi DTBs in FIT
image placed and aligned only by 32bits (4bytes). Based on device tree
specification:
"Specifically, the memory reservation block shall be aligned to an 8-byte boundary
and the structure block to a 4-byte boundary."
is 64bit (8bytes) alignment required. That's why make sure that
fit-dtb.blob and u-boot.itb as our primary target images for Xilinx ZynqMP
are all 64bit aligned.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Reviewed-by: Andre Przywara <andre.przywara@arm.com>