riscv: hw: Drop "clock-frequency" property of cpu nodes

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: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>

include/hw/riscv/sifive_u.h

@@ -75,7 +75,6 @@ enum {
 };
 
 enum {
-    SIFIVE_U_CLOCK_FREQ = 1000000000,
     SIFIVE_U_HFCLK_FREQ = 33333333,
     SIFIVE_U_RTCCLK_FREQ = 1000000
 };