doc: doxygen: define XEN_GUEST_HANDLE_64 as no-op

Doxygen gets confused by the macro expansion in
include/zephyr/xen/public/domctl.h as that is only defined if
CONFIG_ARM64=y, add a no-op expansion of that macro when building the
documention so it has something to work on.

Signed-off-by: Fabio Baltieri <fabiobaltieri@google.com>