arm64: Update the introduction of Xen boot commands in docs/grub.texi

arm64: Update the introduction of Xen boot commands in docs/grub.texi

delete: xen_linux, xen_initrd, xen_xsm
add: xen_module

This update bases on
    commit 0edd750e50698854068358ea53528100a9192902
    Author: Vladimir Serbinenko <phcoder@gmail.com>
    Date:   Fri Jan 22 10:18:47 2016 +0100

        xen_boot: Remove obsolete module type distinctions.

Also bases on the module loading mechanism of Xen code:
488c2a8 docs/arm64: clarify the documention for loading XSM support
67831c4 docs/arm64: update the documentation for loading XSM support
ca32012 xen/arm64: check XSM Magic from the second unknown module.

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