Move conditional compilation of MPC8XXX SPI driver to Makefile

Signed-off-by: Ben Warren <biggerbadderben@gmail.com>
diff --git a/drivers/spi/Makefile b/drivers/spi/Makefile
index e66e0ee..4f7b679 100644
--- a/drivers/spi/Makefile
+++ b/drivers/spi/Makefile
@@ -25,7 +25,7 @@
 
 LIB	:= $(obj)libspi.a
 
-COBJS-y += mpc8xxx_spi.o
+COBJS-$(CONFIG_MPC8XXX_SPI) += mpc8xxx_spi.o
 COBJS-$(CONFIG_ATMEL_SPI) += atmel_spi.o
 COBJS-$(CONFIG_MXC_SPI) += mxc_spi.o