irqchip/ocelot: Add support for Serval platforms

This patch extends irqchip driver for ocelot to be used with an other
vcoreiii base platform: Serval.

Based on a larger patch from Lars Povlsen <lars.povlsen@microchip.com>

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/20201125103206.136498-6-gregory.clement@bootlin.com