qdev-monitor: print the device's clock with info qtree

qdev-monitor: print the device's clock with info qtree

This prints the clocks attached to a DeviceState when using
"info qtree" monitor command. For every clock, it displays the
direction, the name and if the clock is forwarded. For input clock,
it displays also the frequency.

This is based on the original work of Frederic Konrad.

Here follows a sample of `info qtree` output on xilinx_zynq machine
after linux boot with only one uart clocked:
> bus: main-system-bus
>  type System
>  [...]
>  dev: cadence_uart, id ""
>    gpio-out "sysbus-irq" 1
>    clock-in "refclk" freq_hz=0.000000e+00
>    chardev = ""
>    mmio 00000000e0001000/0000000000001000
>  dev: cadence_uart, id ""
>    gpio-out "sysbus-irq" 1
>    clock-in "refclk" freq_hz=1.375661e+07
>    chardev = "serial0"
>    mmio 00000000e0000000/0000000000001000
>  [...]
>  dev: xilinx,zynq_slcr, id ""
>    clock-out "uart1_ref_clk" freq_hz=0.000000e+00
>    clock-out "uart0_ref_clk" freq_hz=1.375661e+07
>    clock-in "ps_clk" freq_hz=3.333333e+07
>    mmio 00000000f8000000/0000000000001000

Signed-off-by: Damien Hedde <damien.hedde@greensocs.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>
Message-id: 20200406135251.157596-10-damien.hedde@greensocs.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>