The "clock-frequency" property of cpu nodes isn't required. Drop it.

This is to keep in sync with Linux kernel commit below:
https://patchwork.kernel.org/patch/11133031/Signed-off-by: NBin Meng <bmeng.cn@gmail.com>
Reviewed-by: NAlistair Francis <alistair.francis@wdc.com>
Signed-off-by: NPalmer Dabbelt <palmer@sifive.com>