drivers: intc: irqstr: manage dispatchers dynamically

Currently, all dispatcher interrupts are enabled during
the driver init() function, which will cause a bus fault
unless the PM domain associated with irqsteer is powered on.

Since PM will be done during irq_enable()/irq_disable(),
add support for dynamically enabling/disabling dispatchers.
This way, the reg. space of the dispatchers will be accessed
when the PM domain is powered on.

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