gpio: grgpio: Add irq support

The drivers sets up an irq domain and hands out unique irqs to irq
capable gpio lines regardless of how underlying irq maps to gpio
lines. Any gpio line can map to any one or none of the irqs of the
core, independently of each other.
Signed-off-by: Andreas Larsson <andreas@gaisler.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

Documentation/devicetree/bindings/gpio/gpio-grgpio.txt

@@ -17,5 +17,10 @@ Optional properties:
 
 - nbits : The number of gpio lines. If not present driver assumes 32 lines.
 
+- irqmap : An array with an index for each gpio line. An index is either a valid
+	index into the interrupts property array, or 0xffffffff that indicates
+	no irq for that line. Driver provides no interrupt support if not
+	present.
+
 For further information look in the documentation for the GLIB IP core library:
 http://www.gaisler.com/products/grlib/grip.pdf