overlays: mcp23017: rename the GPIO pins node with the device

In order to allow the overlay to be loaded multiple times the
GPIO node for the interrupt line needs to be unique.
Rename it based on the MCP23017 I2C address

https://github.com/raspberrypi/linux/issues/3207



Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.org>