arm64: dts: socfpga: agilex: fix dtbs_check warning for f2s-free-clk

The f2s-free-clk requires a clock-frequency value. We put in an
arbitrary value of 100 MHz for a constant. The true clock frequency
would get generated in an FPGA design and the bootloader will populated
in actual hardware designs.

This fixes warning like this:

arch/arm64/boot/dts/intel:34:8
      4  f2s-free-clk (fixed-clock): 'clock-frequency' is a required property

Signed-off-by: Dinh Nguyen <dinguyen@kernel.org>

arch/arm64/boot/dts/intel/socfpga_agilex.dtsi

@@ -126,6 +126,7 @@ cb_intosc_ls_clk: cb-intosc-ls-clk {
 		f2s_free_clk: f2s-free-clk {
 			#clock-cells = <0>;
 			compatible = "fixed-clock";
+			clock-frequency = <100000000>;
 		};
 
 		osc1: osc1 {