drivers/net/fm: descend only when CONFIG_FMAN_ENET=y

CONFIG_FMAN_ENET is defined only for CPU mpc85xx.
We do not need to filter by CPU mpc85xx.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
diff --git a/Makefile b/Makefile
index 0e9609f..5cdfc6d 100644
--- a/Makefile
+++ b/Makefile
@@ -271,9 +271,7 @@
 	drivers/power/pmic/libpmic.o \
 	drivers/power/battery/libbattery.o
 LIBS-y += drivers/spi/libspi.o
-ifeq ($(CPU),mpc85xx)
-LIBS-y += drivers/net/fm/libfm.o
-endif
+LIBS-$(CONFIG_FMAN_ENET) += drivers/net/fm/libfm.o
 LIBS-y += drivers/serial/libserial.o
 LIBS-y += drivers/usb/eth/libusb_eth.o
 LIBS-y += drivers/usb/gadget/libusb_gadget.o