avr32: Call tick_nohz_{stop,restart}_sched_tick() in idle loop

This fixes a hang on boot with nohz enabled. nohz is not actually
supported in mainline yet, but patches that add support for it are
currently under review.

When nohz is compiled out, the functions are no-ops, so this patch
results in no functional change, but it arguably makes the code more
correct.

Signed-off-by: Haavard Skinnemoen <hskinnemoen@atmel.com>