ARM: Fix generic irq chip

As a result of c42321c7 (genirq: Make generic irq chip depend on
CONFIG_GENERIC_IRQ_CHIP), we now need those platforms using this in
my tree to select this symbol.

Acked-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Nicolas Pitre <nico@fluxnic.net>
Acked-by: Sekhar Nori <nsekhar@ti.com>
Acked-by: Kevin Hilman <khilman@ti.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>