irqchip/irq-mvebu-gicp: Add missing spin_lock init

A spin lock is used in the irq-mvebu-gicp driver, but it is never
initialized. This patch adds the missing spin_lock_init() call in the
driver's probe function.

Fixes: a68a63cb ("irqchip/irq-mvebu-gicp: Add new driver for Marvell GICP")
Signed-off-by: Antoine Tenart <antoine.tenart@free-electrons.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by:  <gregory.clement@free-electrons.com>
Acked-by:  <marc.zyngier@arm.com>
Cc: thomas.petazzoni@free-electrons.com
Cc: andrew@lunn.ch
Cc: jason@lakedaemon.net
Cc: nadavh@marvell.com
Cc: miquel.raynal@free-electrons.com
Cc: linux-arm-kernel@lists.infradead.org
Cc: sebastian.hesselbarth@gmail.com
Cc: stable@vger.kernel.org
Link: https://lkml.kernel.org/r/20171025072326.21030-1-antoine.tenart@free-electrons.com