board: ti: am335x: Fix selection of BeagleBone Green
The BeagleBone Green board is a revision of the BoneBlack board.
Having BeagleBone Black devicetree listed before BeagleBone Green will
select always the BeagleBone Black devicetree following the functioning of
board_fit_config_name_match().
Fix it by changing the test condition and selecting BoneBlack board only
if it is not a revision of this board.