kernel: sched: sleep: Use value returned by z_add_timeout

z_tick_sleep function needs to calculate expected wakeup tick. It
required reading current system clock tick. It can be costly since
it is a register access. When timeout is added this value is
calculated and z_add_timeout returns it. It can be used instead to
optimize sleep performance.

Signed-off-by: Krzysztof Chruściński <krzysztof.chruscinski@nordicsemi.no>