sys: timeutil: compute the remainder in timespec to timeout conversion

Since it's possible that rounding up might not always be the right thing
to do in every situation, in order to allow the application to make more
informed decisions, we created a modified timespec_to_timeout() that also
returns the remainder (or difference) between the requested time to
convert and resulting k_timeout_t. The difference is expressed as a
timespec object.

Signed-off-by: Chris Friedt <cfriedt@tenstorrent.com>