ia64: convert to legacy_timer_tick

ia64 is the only architecture that calls xtime_update() in a loop,
once for each jiffie that has passed since the last event.

Before commit 3171a030 ("[PATCH] simplify update_times (avoid
jiffies/jiffies_64 aliasing problem)") in 2006, it could not actually do
this any differently, but now it seems simpler to just pass the number
of jiffies that passed in the meantime.

While this loses the ability process interrupts in the middle of
the timer tick by calling local_irq_enable(), doing so is fairly
peculiar anyway and it seems better to just do what everyone
else does here.

Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>