arm: clarify the use of muldiv64()

arm: clarify the use of muldiv64()

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>