avr32: improve NGW100 I2C/PMBus setup

Basic I2C initialization for the NGW100 board:

  - Provide empty i2c device table. Daughtercards may add devices,
    and the ATtiny24 could do stuff too.

  - Set up EXTINT(3) so the ATtiny24 can interrupt the AP7000.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Haavard Skinnemoen <haavard.skinnemoen@atmel.com>