gpio: adp5588-gpio: support interrupt controller

Implement irq_chip functionality on ADP5588/5587 GPIO expanders.  Only
level sensitive interrupts are supported.  Interrupts provided by this
irq_chip must be requested using request_threaded_irq().

Signed-off-by: Michael Hennerich <michael.hennerich@analog.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/drivers/gpio/Kconfig b/drivers/gpio/Kconfig
index bc7b0fc..3f3181d 100644
--- a/drivers/gpio/Kconfig
+++ b/drivers/gpio/Kconfig
@@ -272,6 +272,13 @@
 	  To compile this driver as a module, choose M here: the module will be
 	  called adp5588-gpio.
 
+config GPIO_ADP5588_IRQ
+	bool "Interrupt controller support for ADP5588"
+	depends on GPIO_ADP5588=y
+	help
+	  Say yes here to enable the adp5588 to be used as an interrupt
+	  controller. It requires the driver to be built in the kernel.
+
 comment "PCI GPIO expanders:"
 
 config GPIO_CS5535