riscv: dts: fu540-c000: drop "timebase-frequency"

On FU540-based systems, the "timebase-frequency" (RTCCLK) is sourced
from an external crystal located on the PCB.  Thus the
timebase-frequency DT property should be defined by the board that
uses the SoC, not the SoC itself.  Drop the superfluous
timebase-frequency property from the SoC DT data.  (It's already
present in the board DT data.)

Signed-off-by: Paul Walmsley <paul.walmsley@sifive.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>