irqchip/armada-370-xp: Use atomic_io_modify() instead of another spinlock

Use the dedicated atomic_io_modify() instead of a open coded spin_lock() +
readl() + writel() + spin_unlock() sequence.

This allows to drop the irq_controller_lock spinlock from the driver.

Signed-off-by: Marek Behún <kabel@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>