pinctrl-mcp23s08: Fix property-name in dt-example

The device-tree properties documentation-file specifies the property
"microchip,spi-present-mask" as required for MCP23SXX chips. However,
the device-tree-source example below it uses only "spi-present-mask".
Without "microchip," on the front, the driver will print "missing
spi-present-mask" when it initializes.

Update the device-tree example with the correct property-name.
Signed-off-by: Peter Vernia <peter.vernia@gmail.com>
Reviewed-by: Jan Kundrát <jan.kundrat@cesnet.cz>
Signed-off-by: Rob Herring <robh@kernel.org>

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

@@ -82,7 +82,7 @@ gpiom1: gpio@0 {
         compatible = "microchip,mcp23s17";
         gpio-controller;
         #gpio-cells = <2>;
-        spi-present-mask = <0x01>;
+        microchip,spi-present-mask = <0x01>;
         reg = <0>;
         spi-max-frequency = <1000000>;
 };