drivers: intc: irqstr: add PM support

Add support for PM. The strategy is as follows:

	1) For level 1 interrupts: don't care, these don't
	require the PM domain of irqsteer to be turned on
	since they are, well, direct.

	2) For level 2 interrupts: use the reference count
	of the dispatchers.

Upon doing a get() on a dispatcher with its reference count
set to 0, before enabling the IRQ (meaning accessing the
reg. space) increment the reference count of the irqstr device
(which will result in the PM domain being enabled if 0).

Upon doin a put() on a dispatcher with its reference count
set to 1, after disabling the IRQ (meaning accessing the
reg. space) decrement the reference count of the irqstr device
(which will result in the PM domain being disabled if 0).

In summary, the PM domain of the device will be enabled if
at least one dispatcher is in use. On the other hand, the
PM domain of the device will be disabled if there's no
dispatchers in use (assuming there's no other dependencies).

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