clock: Add clock_ns_to_ticks() function

Add a clock_ns_to_ticks() function which does the opposite of
clock_ticks_to_ns(): given a duration in nanoseconds, it returns the
number of clock ticks that would happen in that time.  This is useful
for devices that have a free running counter register whose value can
be calculated when it is read.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Luc Michel <luc@lmichel.fr>
Reviewed-by: Hao Wu <wuhaotsh@google.com>
Tested-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-id: 20210219144617.4782-4-peter.maydell@linaro.org

docs/devel/clocks.rst

@@ -360,6 +360,18 @@ rather than simply passing it to a QEMUTimer function like
 ``timer_mod_ns()`` then you should be careful to avoid overflow
 in those calculations, of course.)
 
+Obtaining tick counts
+---------------------
+
+For calculations where you need to know the number of ticks in
+a given duration, use ``clock_ns_to_ticks()``. This function handles
+possible non-whole-number-of-nanoseconds periods and avoids
+potential rounding errors. It will return '0' if the clock is stopped
+(i.e. it has period zero). If the inputs imply a tick count that
+overflows a 64-bit value (a very long duration for a clock with a
+very short period) the output value is truncated, so effectively
+the 64-bit output wraps around.
+
 Changing a clock period
 -----------------------