ARM: dts: rainier: Describe GPIO mux on I2C3

We have a 4-bus mux whose output is selected by two GPIO inputs. Wire it
up in the devicetree and ensure the output is enabled by hogging the
appropriate line.

Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
Signed-off-by: Joel Stanley <joel@jms.id.au>