If system clock runs fast k_cycle_get_32(), which is the timestamp
source, wraps often (few minutes). This patch changes default
timestamp function to use k_uptime_get_32() if system clock
frequency is higher than 1 MHz and k_cycle_get_32() otherwise.
If system clock runs at 1 MHz, counter will wrap every 71.5 minutes.
Signed-off-by: Krzysztof Chruscinski <krzysztof.chruscinski@nordicsemi.no>