avr32: add ATAG_BOARDINFO

This patch adds a new ATAG_BORADINFO to U-Boot. This tag is intended to hand
over the bd->bi_board_number to the linux kernel for early stage board
information like a board revision or other kind of board specific decisions
necessary before the linux peripherial drivers are up.

Signed-off-by: Andreas Bießmann <biessmann@corscience.de>