So far the ICU only handled NSR interrupts through GICP. An SEI driver
provides an MSI domain through which it is possible to raise SEI, so
let's add SEI support to the ICU driver.

Handle the NSR probe function in a more generic way to support other
type of interrupts.

Each interrupt domain is a tree domain to avoid allocation the 207
entries each time.
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>