The ARM PMU node is described enough with first compatible so remove the
arm,armv8-pmuv3 to fix dtschema warnings like:

    arm-pmu: compatible: Additional items are not allowed ('arm,armv8-pmuv3' was unexpected)
    arm-pmu: compatible: ['arm,cortex-a57-pmu', 'arm,armv8-pmuv3'] is too long
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Reviewed-by: Alim Akhtar <alim.akhtar@samsung.com>
Tested-by: Alim Akhtar <alim.akhtar@samsung.com>