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>