irqchip/irq-mvebu-icu: Disociate ICU and NSR

NSR (non-secure interrupts) are handled in the ICU driver like if there
was only this type of interrupt in the ICU. Change this behavior to
prepare the introduction of SEI (System Error Interrupts) support by
moving the NSR code in a separate function. This is done under the form
of a 'probe' function to ease future migration to NSR/SEI being platform
devices part of the ICU. The 'icu' structure is passed as driver data
and not as a parameter for the same reason.

Reviewed-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>