arm/timer: fix panic when booting with DT

arm/timer: fix panic when booting with DT

While to support ACPI, patch "arm/acpi: Parse GTDT to initialize timer"
refactors the functions preinit_xen_time and init_xen_time. But it
wrongly moves the platform_get_irq from init_xen_time to
preinit_dt_xen_time and this will cause booting failure.

So move platform_get_irq back to init_xen_time to fix it.

Signed-off-by: Shannon Zhao <shannon.zhao@linaro.org>
Reviewed-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>