MIPS: Netlogic: handle modular AHCI builds

Commits a9514409 ("MIPS: Netlogic: Support for XLP3XX on-chip SATA")
and fedfcb11 ("MIPS: Netlogic: XLP9XX on-chip SATA support") added
ahci-init and ahci-init-xlp2 as objects to build when CONFIG_SATA_AHCI
is enabled.

If CONFIG_SATA_AHCI is made modular, these two files will also get built
as modules (obj-m), which will result in the following linking failure:

ERROR: "nlm_set_pic_extra_ack" [arch/mips/netlogic/xlp/ahci-init.ko]
undefined!
ERROR: "nlm_io_base" [arch/mips/netlogic/xlp/ahci-init.ko] undefined!
ERROR: "nlm_nodes" [arch/mips/netlogic/xlp/ahci-init-xlp2.ko] undefined!
ERROR: "nlm_set_pic_extra_ack"
[arch/mips/netlogic/xlp/ahci-init-xlp2.ko] undefined!
ERROR: "xlp_socdev_to_node" [arch/mips/netlogic/xlp/ahci-init-xlp2.ko]
undefined!
ERROR: "nlm_io_base" [arch/mips/netlogic/xlp/ahci-init-xlp2.ko]
undefined!

Just check whether CONFIG_SATA_AHCI is defined for this build, and if
that is the case, add these objects to the list of built-in object
files.
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Cc: linux-mips@linux-mips.org
Cc: ganesanr@broadcom.com
Cc: jchandra@broadcom.com
Patchwork: https://patchwork.linux-mips.org/patch/7855/Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/netlogic/xlp/Makefile

@@ -4,5 +4,7 @@ ifdef CONFIG_USB
 obj-y				+= usb-init.o
 obj-y				+= usb-init-xlp2.o
 endif
-obj-$(CONFIG_SATA_AHCI)		+= ahci-init.o
-obj-$(CONFIG_SATA_AHCI)		+= ahci-init-xlp2.o
+ifdef CONFIG_SATA_AHCI
+obj-y				+= ahci-init.o
+obj-y				+= ahci-init-xlp2.o
+endif