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

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>