dt-bindings: pinctrl: mcp23s08: add documentation for drive-open-drain

This flag set the mcp23s08 device irq type to open drain active low.
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Phil Reid <preid@electromag.com.au>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

Documentation/devicetree/bindings/pinctrl/pinctrl-mcp23s08.txt

@@ -45,6 +45,8 @@ Optional properties:
   - first cell is the pin number
   - second cell is used to specify flags.
 - interrupt-controller: Marks the device node as a interrupt controller.
+- drive-open-drain: Sets the ODR flag in the IOCON register. This configures
+        the IRQ output as open drain active low.
 
 Optional device specific properties:
 - microchip,irq-mirror: Sets the mirror flag in the IOCON register. Devices