irqchip/mvebu-pic: New driver for Marvell Armada 7K/8K PIC

The Marvell Armada 7K/8K integrates a secondary interrupt controller
very originally named "PIC". It is connected to the main GIC via a
PPI. Amongst other things, this PIC is used for the ARM PMU.

This commit adds a simple irqchip driver for this interrupt
controller. Since this interrupt controller is not needed early at boot
time, we make the driver a proper platform driver rather than use the
IRQCHIP_DECLARE() mechanism.

Signed-off-by: Yehuda Yitschak <yehuday@marvell.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Link: https://lkml.kernel.org/r/1470408921-447-3-git-send-email-thomas.petazzoni@free-electrons.com
Signed-off-by: Jason Cooper <jason@lakedaemon.net>