sched tickless: provide SCHED_TICKLESS_TIMER option

As discussed in #1710, when an architecture decides to support the system timer using the arch_timer/arch_alarm drivers, this requires selecting the matching option for tickless (either SCHED_TICKLESS_ALARM defined or not). To make this automatic (without user facing a build error and having to fix it), it is better to introduce a SCHED_TICKLESS_TIMER option (selected via a `choice` in Kconfig) so that when ARCH_TIMER is selected and SCHED_TICKLESS is defined, SCHED_TICKLESS_TIMER is select as well. This is already being done for ARCH_ALARM and SCHED_TICKLESS_ALARM, as defined in #1710.