hw/core/clock: trace clock values in Hz instead of ns

hw/core/clock: trace clock values in Hz instead of ns

The nanosecond unit greatly limits the dynamic range we can display in
clock value traces, for values in the order of 1GHz and more. The
internal representation can go way beyond this value and it is quite
common for today's clocks to be within those ranges.

For example, a frequency between 500MHz+ and 1GHz will be displayed as
1ns. Beyond 1GHz, it will show up as 0ns.

Replace nanosecond periods traces with frequencies in the Hz unit
to have more dynamic range in the trace output.

Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Damien Hedde <damien.hedde@greensocs.com>
Signed-off-by: Luc Michel <luc@lmichel.fr>
Tested-by: Guenter Roeck <linux@roeck-us.net>
Tested-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>