drivers/timer/apic_timer: move to periodic mode

This driver is impossible to make time-accurate using single-shot
mode. Time accuracy may be obtained only by using periodic mode, meaning
it is not tickless capable either. Let's simplify the code by only
supporting periodic mode and strip out the TSC stuff. Any hardware with
TSC capability should now use the apic-tsc driver instead.

Signed-off-by: Nicolas Pitre <npitre@baylibre.com>