arm64: dts: marvell: add AP806 SEI subnode

Add the System Error Interrupt node, representing an IRQ chip which is
part of the GIC. The SEI node aggregates interrupts from the AP through
wired interrupts, and from the CPs through MSIs.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Signed-off-by: Gregory CLEMENT <gregory.clement@bootlin.com>