bcm63xx: drop target

bcm63xx: drop target

BCM63xx maintainance is a PITA since there's no full Device Tree support and
therefore every board needs it's own definition in board_bcm963xx.c
There's no DSA support for this board and there have been very few
contributions in the last years.
Moreover, BCM63xx SoCs >= BCM6358 are already supported in bmips target, so
any efforts to keep this target alive aren't worth the time.
Let's focus the community efforts on improving bmips instead.

Signed-off-by: Álvaro Fernández Rojas <noltari@gmail.com>