util/grub.d/20_linux_xen.in: Add xen_boot command support for aarch64

util/grub.d/20_linux_xen.in: Add xen_boot command support for aarch64

This patch adds the support of xen_boot command for aarch64:
    xen_hypervisor
    xen_module
These two commands are only for aarch64, since it has its own protocol and
commands to boot xen hypervisor and Dom0, but not multiboot.

For other architectures, they are still using multiboot and module
commands.

Signed-off-by: Fu Wei <fu.wei@linaro.org>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>