irqchip/mxs: Add missing set_handle_irq()

The rework of the driver missed to move the call to set_handle_irq() into
asm9260_of_init(). As a consequence no interrupt entry point is installed and
no interrupts are delivered

Solution is simple: Install the interrupt entry handler.

Fixes: 7e4ac676

 ("irqchip/mxs: Add Alphascale ASM9260 support")
Signed-off-by: Oleksij Rempel <linux@rempel-privat.de>
Cc: kernel@pengutronix.de
Cc: jason@lakedaemon.net
Cc: marc.zyngier@arm.com
Cc: stable@vger.kernel.org
Link: http://lkml.kernel.org/r/1454061473-24957-1-git-send-email-linux@rempel-privat.de
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>