MIPS: Loongson: Hide the pci code behind CONFIG_PCI

MIPS: Loongson: Hide the pci code behind CONFIG_PCI

The pci.c code depends on symbols which are only visible
if CONFIG_PCI is selected.

Also fixes the following problem on loongson allnoconfig:
arch/mips/built-in.o: In function `pcibios_init':
pci.c:(.init.text+0x528):
undefined reference to `register_pci_controller'
arch/mips/built-in.o:(.data+0xc):
undefined reference to `loongson_pci_ops'

Signed-off-by: Markos Chandras <markos.chandras@imgtec.com>
Acked-by: Steven J. Hill <Steven.Hill@imgtec.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/5584/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>