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>