Documentation: arm64: pmu: Add Broadcom Vulcan PMU binding

Document the compatible string for Broadcom Vulcan PMU.
Also arranged the list in alphabetical order.
Signed-off-by: Ashok Kumar <ashoks@broadcom.com>
Acked-by: Rob Herring <robh@kernel.org>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>

Documentation/devicetree/bindings/arm/pmu.txt

@@ -22,10 +22,11 @@ Required properties:
 	"arm,arm11mpcore-pmu"
 	"arm,arm1176-pmu"
 	"arm,arm1136-pmu"
+	"brcm,vulcan-pmu"
+	"cavium,thunder-pmu"
 	"qcom,scorpion-pmu"
 	"qcom,scorpion-mp-pmu"
 	"qcom,krait-pmu"
-	"cavium,thunder-pmu"
 - interrupts : 1 combined interrupt or 1 per core. If the interrupt is a per-cpu
                interrupt (PPI) then 1 interrupt should be specified.