Remove BTB_ENABLE from proc-xsc3.S

On some early revisions of xsc3 enabling the branch target buffer can cause
crashes, see erratum #42.

Cc: Lennert Buytenhek <buytenh@wantstofly.org>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>