kernel: Refactor remaining time evaluation for timeouts

Factor out the code for evaluating the remaining time for _timeout
structs so that it can also be used for other objects besides k_timer
structs (like k_delayed_work, coming in a subsequent patch).

Change-Id: I243a7b29fb2831f06e95086a31f0d3a6c37dad67
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>
2 files changed