Documentation: arm: pmu: Add Potenza PMU binding

This patch documents the compatible string for APM X-Gene Potenza CPU's PMU.
Signed-off-by: NVinayak Kale <vkale@apm.com>
Signed-off-by: NFeng Kan <fkan@apm.com>

Documentation/devicetree/bindings/arm/pmu.txt

@@ -7,6 +7,7 @@ representation in the device tree should be done as under:-
 Required properties:
 
 - compatible : should be one of
+	"apm,potenza-pmu"
 	"arm,armv8-pmuv3"
 	"arm,cortex-a17-pmu"
 	"arm,cortex-a15-pmu"