xen/arm: Disable timers and release their interrupts on CPU hot-unplug

xen/arm: Disable timers and release their interrupts on CPU hot-unplug

When a CPU is hot-unplugged we need to disable timers and release
their interrupts in order to free the memory that was allocated when
interrupts were requested (using request_irq()). The request_irq()
is called for each timer interrupt when the CPU gets hotplugged
(start_secondary->init_timer_interrupt->request_irq).
With this patch timers will be disabled and interrupts will be
released when the newly added callback receives CPU_DYING event.

Signed-off-by: Mirela Simonovic <mirela.simonovic@aggios.com>
Acked-by: Julien Grall <julien.grall@arm.com>