irqchip: armada-370-xp: implement MSI support

This commit introduces the support for the MSI interrupts in the
armada-370-xp interrupt controller driver. It registers an MSI chip to
the MSI chip registry, which will be used by the Marvell PCIe host
controller driver.

The MSI interrupts use the 16 high doorbells, and are therefore
notified using IRQ1 of the main interrupt controller.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Signed-off-by: Jason Cooper <jason@lakedaemon.net>