xen/arm: timer: replace timer_dt_irq by timer_get_irq

xen/arm: timer: replace timer_dt_irq by timer_get_irq

The function is nearly only used to retrieve the IRQ number.

There is one place where the IRQ type is used (in domain_build.c) but
as the timer IRQ is virtualised for guest we might not have the same property
(e.g active-low level sensitive interrupt).

Replace timer_dt_irq by timer_get_irq which will return the IRQ number.

Signed-off-by: Julien Grall <julien.grall@linaro.org>
Acked-by: Ian Campbell <ian.campbell@citrix.com>