riscv: dts: starfive: visionfive 2: use cpus label for timebase freq

As pointed out by Krzysztof "Board should not bring new CPU nodes.
Override by label instead."

Suggested-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Jisheng Zhang <jszhang@kernel.org>
Reviewed-by: Emil Renner Berthing <emil.renner.berthing@canonical.com>
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>