irqchip/irq-mvebu-icu: Clarify the reset operation of configured interrupts

Rewrite a small section to clarify the reset operation of interrupts
already configured by ATF that we want to handle in the driver. This
will simplify the introduction of System Error Interrupts support.

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>