clock: Define and use new clock_display_freq()

clock: Define and use new clock_display_freq()

It's common to want to print a human-readable indication of a clock's
frequency. Provide a utility function in the clock API to return a
string which is a displayable representation of the frequency,
and use it in qdev-monitor.c.

Before:

  (qemu) info qtree
  [...]
  dev: xilinx,zynq_slcr, id ""
    clock-in "ps_clk" freq_hz=3.333333e+07
    mmio 00000000f8000000/0000000000001000

After:

  dev: xilinx,zynq_slcr, id ""
    clock-in "ps_clk" freq_hz=33.3 MHz
    mmio 00000000f8000000/0000000000001000

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Tested-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Luc Michel <luc@lmichel.fr>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20201215150929.30311-5-peter.maydell@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>