sys: time_units: Add Kconfig option for algorithm selection

Add maximum timeout used for conversion to Kconfig. Option is used
to determine which conversion algorithm to use: faster but overflowing
earlier or slower without early overflow.

Signed-off-by: Krzysztof Chruscinski <krzysztof.chruscinski@nordicsemi.no>
(cherry picked from commit 50c7c7b1e4027b08a594f108df5d62bce1fc229f)