[AVR32] Wire up i2c-gpio on the ATNGW100 board

The NGW100 has a board controller which is hooked up to the TWI lines
on AP7000. Since the TWI driver isn't in mainline, use the i2c-gpio
driver in the mean time.

Signed-off-by: Haavard Skinnemoen <hskinnemoen@atmel.com>