irqchip/irq-mvebu-gicp: Add new driver for Marvell GICP

This commit adds a simple driver for the Marvell GICP, a hardware unit
that converts memory writes into GIC SPI interrupts. The driver provides
a number of functions to the ICU driver to allocate GICP interrupts, and
get the physical addresses that the ICUs should write to to set/clear
interrupts.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reviewed-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>