pxa2xx_timer: separate irq for pxa27x handling

First, sysbus_init_irq shan't be called on on-stack variables. Indeed,
it only stores a passed pointer in qdev and the stored irq is later
populated, so we get a nice write-to-stack bug.
Second, irq for pxa27x should probably be handled in a more gentler way,
as we should check if we have events to raise this irq.

Signed-off-by: Dmitry Eremin-Solenikov <dbaryshkov@gmail.com>
Signed-off-by: Andrzej Zaborowski <andrew.zaborowski@intel.com>
