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>