genirq: Provide irq_gc_{lock_irqsave,unlock_irqrestore}() helpers

Some irqchip drivers need to take the generic chip lock outside of the
irq context.

Provide the irq_gc_{lock_irqsave,unlock_irqrestore}() helpers to allow
one to disable irqs while entering a critical section protected by
gc->lock.

Note that we do not provide optimized version of these helpers for !SMP,
because they are not called from the hot-path.

[ tglx: Added a comment when these helpers should be [not] used ]

Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Cc: Jason Cooper <jason@lakedaemon.net>
Cc: Marc Zyngier <marc.zyngier@arm.com>
Cc: Nicolas Ferre <nicolas.ferre@atmel.com>
Cc: stable@vger.kernel.org
Cc: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Link: http://lkml.kernel.org/r/1473775109-4192-1-git-send-email-boris.brezillon@free-electrons.com


Signed-off-by: Thomas Gleixner <tglx@linutronix.de>