mmc: descend into drivers/mmc only when CONFIG_MMC is enabled

This simplifies makefiles.  Also, arrange the order of objects in
drivers/mmc/Makefile so that the framework objects are listed before
drivers.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
diff --git a/drivers/Makefile b/drivers/Makefile
index 6916420..64c39d3 100644
--- a/drivers/Makefile
+++ b/drivers/Makefile
@@ -76,6 +76,7 @@
 obj-$(CONFIG_FPGA) += fpga/
 obj-y += hwmon/
 obj-y += misc/
+obj-$(CONFIG_MMC) += mmc/
 obj-y += pcmcia/
 obj-y += dfu/
 obj-$(CONFIG_X86) += pch/