avr32: pass i2c board info through at32_add_device_twi

New-style I2C drivers require that motherboard-mounted I2C devices are
registered with the I2C core, typically at arch_initcall time.  This
can be done nice and neat by passing the struct i2c_board_info[]
through at32_add_device_twi just like we do for the SPI board info.

While we've got the hood up, remove a duplicate declaration of
at32_add_device_twi() in board.h.

[hskinnemoen@atmel.com: add missing i2c_board_info forward-declaration]
Signed-Off-By: Ben Nizette <bn@niasdigital.com>
Signed-off-by: Haavard Skinnemoen <hskinnemoen@atmel.com>