gpio: pxa: fix Documentation of interrupt-names property

The property name to pass names for interrupt resources is called
"interrupt-names", not "interrupt-name". Fix it to avoid further
confusion.

Signed-off-by: Daniel Mack <[e-mail]>
Cc: Haojian Zhuang <[e-mail]>
Cc: Linus Walleij <[e-mail]>
Cc: Arnd Bergmann <[e-mail]>
Signed-off-by: Linus Walleij <[e-mail]>

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

···
   There're three gpio interrupts in arch-pxa, and they're gpio0,
   gpio1 and gpio_mux. There're only one gpio interrupt in arch-mmp,
   gpio_mux.
-- interrupt-name : Should be the name of irq resource. Each interrupt
-  binds its interrupt-name.
+- interrupt-names : Should be the names of irq resources. Each interrupt
+  uses its own interrupt name, so there should be as many interrupt names
+  as referenced interrups.
 - interrupt-controller : Identifies the node as an interrupt controller.
 - #interrupt-cells: Specifies the number of cells needed to encode an
   interrupt source.
···
 		compatible = "marvell,mmp-gpio";
 		reg = <0xd4019000 0x1000>;
 		interrupts = <49>;
-		interrupt-name = "gpio_mux";
+		interrupt-names = "gpio_mux";
 		gpio-controller;
 		#gpio-cells = <1>;
 		interrupt-controller;