sim/periodic: allow events to be enabled and disabled.
sim/cpu: initialize event list early.

We need it initialized before we start adding any peripherals otherwise
we'll use an uninitialized list head.
sim: partial timer implementation.

No support for interrupts from the timers yet, but basic support for
one-shot and periodic timers.
rtl/keynsham: implement timers without IRQ support.
rtl/irq: fixup lint errors.