remove Broadcom Northstar 2 Target entry

The Broadcom Northstar 2 support was removed when the
bcm958712k board was removed but the target entry was
missed so clean that up as well.

Fixes: d59bc09d ("arm: Remove bcm958712k board")
Signed-off-by: Peter Robinson <pbrobinson@gmail.com>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>