[MIPS] cpu/mips/Makefile: Split [CS]OBJS onto separate lines

Also get rid of some #ifdefs in *.c files.

Signed-off-by: Shinya Kuribayashi <skuribay@ruby.dti.ne.jp>
diff --git a/cpu/mips/au1x00_usb_ohci.c b/cpu/mips/au1x00_usb_ohci.c
index e03b125..1ca8aaf 100644
--- a/cpu/mips/au1x00_usb_ohci.c
+++ b/cpu/mips/au1x00_usb_ohci.c
@@ -35,7 +35,7 @@
 
 #include <config.h>
 
-#if defined(CONFIG_SOC_AU1X00) && defined(CONFIG_USB_OHCI)
+#ifdef CONFIG_USB_OHCI
 
 /* #include <pci.h> no PCI on the AU1x00 */