drivers: intc: irqstr: add reference count for IRQs

Currently, shared interrupts pose a big problem because
irq_disable() doesn't keep track of the number of clients
using that interrupt line. As such, add a reference count
mechanism which will stop the interrupt from being disabled
if there's still clients using it.

Signed-off-by: Laurentiu Mihalcea <laurentiu.mihalcea@nxp.com>