docs/arm64: update the documentation for loading XSM support

This patch updates the documentation for allowing detection of an XSM
module that lacks a specific compatible string.
This mechanism has been added by the commit
ca32012341.

Signed-off-by: Fu Wei <fu.wei@linaro.org>
Signed-off-by: Stefano Stabellini <sstabellini@kernel.org>
Acked-by: Julien Grall <julien.grall@arm.com>
Release-acked-by: Wei Liu <wei.liu2@citrix.com>

docs/misc/arm/device-tree/booting.txt

@@ -24,10 +24,24 @@ Each node contains the following properties:
 	string (which must always be present).
 
 	Xen will assume that the first module which lacks a more
-	specific compatible string is a "multiboot,kernel" and that
-	the second such is a "multiboot,ramdisk". Any subsequent
-	modules which lack a specific compatiblity string will not
-	receive any special treatment.
+	specific compatible string is a "multiboot,kernel".
+
+	Xen will check all the modules for the XSM Magic from the second
+	module that lacks a specific compatible string. According to the
+	result of the detection:
+	- if it's an XSM, Xen will assume its compatible string is
+	  "xen,xsm-policy";
+	- if it's not an XSM, for the second module that lacks a specific
+	  compatible string, Xen will assume its compatible string is
+	  "multiboot,ramdisk"; the third and subsequent modules that
+	  lack a specific compatible string will not receive any special
+	  treatment.
+	This means that if the ramdisk module is present and does not have
+	the compatible string "multiboot,ramdisk", then it must always be
+	the second module.
+	Note: This XSM Magic detection behavior was introduced by Xen 4.7.
+	Xen 4.6 (and downwards) still requires the XSM module to have the
+	compatible string "xen,xsm-policy".
 
 	Xen 4.4 supported a different set of legacy compatible strings
 	which remain supported such that systems supporting both 4.4