irqchip/ocelot: Add support for Luton platforms

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

For this platform there is a few differences:
   - the interrupt must be enabled for the parent controller
   - there is no trigger register needed to be managed

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/20201125103206.136498-5-gregory.clement@bootlin.com