arm64: dts: Add Broadcom Vulcan PMU in dts

Add "brcm,vulcan-pmu" compatible string for Broadcom Vulcan PMU.

Signed-off-by: Ashok Kumar <ashoks@broadcom.com>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>