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: NPaul Walmsley <paul.walmsley@sifive.com>
Reviewed-by: NBin Meng <bmeng.cn@gmail.com>

arch/riscv/boot/dts/sifive/fu540-c000.dtsi

@@ -21,7 +21,6 @@
 	cpus {
 		#address-cells = <1>;
 		#size-cells = <0>;
-		timebase-frequency = <1000000>;
 		cpu0: cpu@0 {
 			compatible = "sifive,e51", "sifive,rocket0", "riscv";
 			device_type = "cpu";