irqchip/armada-370-xp: Add Kconfig option for the driver

Instead of building the irq-armada-370-xp driver directly when
CONFIG_ARCH_MVEBU is enabled, this commit introduces an intermediate
CONFIG_ARMADA_370_XP_IRQ hidden Kconfig option.

This allows this option to select other interrupt-related Kconfig
options (which will be needed in follow-up commits) rather than having
such selects done from arch/arm/mach-<foo>/.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Link: https://lkml.kernel.org/r/1455115621-22846-2-git-send-email-thomas.petazzoni@free-electrons.com


Signed-off-by: Jason Cooper <jason@lakedaemon.net>