mips: pci: Add ifdef around pci_proc_domain

mips: pci: Add ifdef around pci_proc_domain

Without these, there are multiple definitions of pci_proc_domain()
and pci_domain_nr() if linux/pci.h and asm/pci.h are included.

Add #ifdefs around them

Signed-off-by: Zubair Lutfullah Kakakhel <Zubair.Kakakhel@imgtec.com>
Reviewed-by: Markos Chandras <Markos.Chandras@imgtec.com>
Cc: Markos.Chandras@imgtec.com
Cc: linux-mips@linux-mips.org
Cc: linux-kernel@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/8670/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>