MIPS: Malta: Do not build the malta-amon.c file if CMP is not enabled

The malta-amon.c file provides functions to access the YAMON Monitoring
interface to bring up secondary VPEs in case of SMP/CMP. As a
result of which, there is no need to build it if CMP is not used.

Signed-off-by: Markos Chandras <markos.chandras@imgtec.com>
Reviewed-by: Paul Burton <paul.burton@imgtec.com>
Cc: linux-mips@linux-mips.org
Patchwork: http://patchwork.linux-mips.org/patch/7993/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>