muldiv64() is used to convert microseconds into CPU ticks.
But it is not clear and not commented. This patch uses macro
to clearly identify what is used: time, CPU frequency and ticks.
For an elapsed time and a given frequency, we compute how many ticks
we have.
Signed-off-by: Laurent Vivier <lvivier@redhat.com> Reviewed-by: Peter Crosthwaite <crosthwaite.peter@gmail.com> Acked-by: Peter Maydell <peter.maydell@linaro.org>