irqchip/ocelot: prepare to support more SoC

This patch extends irqchip driver for oceleot to be used with other
vcoreiii base platforms.

Signed-off-by: Gregory CLEMENT <gregory.clement@bootlin.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Acked-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20201125103206.136498-4-gregory.clement@bootlin.com