avr32: use board_early_init_r instead of board_init_info

Replace the avr32-specific board_init_info hook by the standard
board_early_init_r hook and make it optional.

board_early_init_r() runs somewhat earlier than board_init_info used to
do, but this isn't a problem for any of the in-tree boards.

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