arm: remove ARCH_BCM_63XX config symbol

Commit c4ea7afc accidentally added back a
symbol (CONFIG_ARCH_BCM_63XX) that had been removed upstream.

Fixes: c4ea7afc ("Improve __copy_to_user and __copy_from_user performance")
Signed-off-by: Álvaro Fernández Rojas <noltari@gmail.com>