documentaion: DT: allow a A5 compatible string in global timer

The global timer is present on the Cortex A5. Add a compatibility
string to the DT binding to allow a Cortex A5 global timer.
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Matthew Leach <matthew.leach@arm.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>

Documentation/devicetree/bindings/arm/global_timer.txt

@@ -4,8 +4,11 @@
 
 ** Timer node required properties:
 
-- compatible : Should be "arm,cortex-a9-global-timer"
-		Driver supports versions r2p0 and above.
+- compatible : should contain
+	     * "arm,cortex-a5-global-timer" for Cortex-A5 global timers.
+	     * "arm,cortex-a9-global-timer" for Cortex-A9 global
+	         timers or any compatible implementation. Note: driver
+	         supports versions r2p0 and above.
 
 - interrupts : One interrupt to each core