[AVR32] Implement more at32_add_device_foo() functions

Implement functions for adding platform devices for TWI, MCI, AC97C
and ABDAC. They may need to be modified to cope with platform data,
etc. when the corresponding drivers are ready to be merged, but such
changes are much less likely to conflict than adding support for a
whole new type of device.

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