dt-bindings: Update Documentation/arm references

The Arm documentation has moved to Documentation/arch/arm; update
one devicetree reference to match.

Cc: Krzysztof Kozlowski <krzysztof.kozlowski+dt@linaro.org>
Cc: devicetree@vger.kernel.org
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/devicetree/bindings/arm/xen.txt

@@ -56,7 +56,7 @@ hypervisor {
 };
 
 The format and meaning of the "xen,uefi-*" parameters are similar to those in
-Documentation/arm/uefi.rst, which are provided by the regular UEFI stub. However
+Documentation/arch/arm/uefi.rst, which are provided by the regular UEFI stub. However
 they differ because they are provided by the Xen hypervisor, together with a set
 of UEFI runtime services implemented via hypercalls, see
 http://xenbits.xen.org/docs/unstable/hypercall/x86_64/include,public,platform.h.html.