docs/system: Sort Arm board index into alphabetical order

Sort the board index into alphabetical order.  (Note that we need to
sort alphabetically by the title text of each file, which isn't the
same ordering as sorting by the filename.)

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Message-id: 20200507151819.28444-3-peter.maydell@linaro.org

docs/system/target-arm.rst

@@ -67,19 +67,24 @@ Unfortunately many of the Arm boards QEMU supports are currently
 undocumented; you can get a complete list by running
 ``qemu-system-aarch64 --machine help``.
 
+..
+   This table of contents should be kept sorted alphabetically
+   by the title text of each file, which isn't the same ordering
+   as an alphabetical sort by filename.
+
 .. toctree::
    :maxdepth: 1
 
    arm/integratorcp
-   arm/versatile
    arm/realview
-   arm/xscale
-   arm/palm
-   arm/nseries
-   arm/stellaris
+   arm/versatile
    arm/musicpal
-   arm/sx1
+   arm/nseries
    arm/orangepi
+   arm/palm
+   arm/xscale
+   arm/sx1
+   arm/stellaris
 
 Arm CPU features
 ================