arm64: dts: hi3660: improve pmu description

cortex-a73 pmu driver is supported now. hi3660 is 4*a73 + 4*a53, so it
should use "cortex-a73-pmu" and "cortex-a53-pmu" instead of "armpmu-v3",
then we can use the a73 and a53 events in perf tool directly.

Signed-off-by: Xu YiPing <xuyiping@hisilicon.com>
Signed-off-by: Wei Xu <xuwei5@hisilicon.com>