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/common/spl/Kconfig b/common/spl/Kconfig
index 0d56f10..e82a879 100644
--- a/common/spl/Kconfig
+++ b/common/spl/Kconfig
@@ -729,7 +729,7 @@
 
 config TPL_MMC_SUPPORT
 	bool "Support MMC"
-	depends on TPL
+	depends on TPL && MMC
 	help
 	  Enable support for MMC within TPL. See SPL_MMC_SUPPORT for details.