xen:arm: Populate arm64 image header

xen:arm: Populate arm64 image header

This patch adds image size and flags to XEN image header. It uses
those fields according to the updated Linux kernel image definition.

With this patch bootloader can now place XEN image anywhere in system
RAM at 2MB aligned address without to worry about relocation.
For instance, it fixes the XEN boot on Amlogic SoC where bootloader(U-BOOT)
always relocates the XEN image to an address range reserved for firmware data.

Signed-off-by: Amit Singh Tomar <amittomer25@gmail.com>
Reviewed-by: Andre Pryzwara <andre.przywara@arm.com>
Acked-by: Julien Grall <julien.grall@arm.com>