dt-bindings/interrupt-controller: Add documentation for Marvell SEI controller

Describe the System Error Interrupt (SEI) controller. It aggregates two
types of interrupts, wired and MSIs from respectively the AP and the
CPs, into a single SPI interrupt.

Suggested-by: Haim Boot <hayim@marvell.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>