mmc: complete unfinished move of CONFIG_MMC

Commit 7a777f6d6f35 ("mmc: Add generic Kconfig option") created
a Kconfig entry for this option without any actual moves, then
commit 44c798799f66 ("sunxi: Use Kconfig CONFIG_MMC") moved
instances only for SUNXI.

We generally do not like such partial moves.  This kind of work
is automated by tools/moveconfig.py, so it is pretty easy to
complete this move.

I am adding "default ARM || PPC || SANDBOX" (suggested by Tom).
This shortens the configs and will ease new board porting.

This commit was created as follows:

[1] Edit Kconfig (remove the "depends on", add the "default",
    copy the prompt and help message from Linux)

[2] Run 'tools/moveconfig.py -y -s -r HEAD MMC'

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Jaehoon Chung <jh80.chung@samsung.com>
diff --git a/configs/mx31pdk_defconfig b/configs/mx31pdk_defconfig
index a31ad5b..bb1f121 100644
--- a/configs/mx31pdk_defconfig
+++ b/configs/mx31pdk_defconfig
@@ -3,6 +3,7 @@
 CONFIG_SPL_LIBGENERIC_SUPPORT=y
 CONFIG_SPL_NAND_SUPPORT=y
 CONFIG_SPL_SERIAL_SUPPORT=y
+# CONFIG_MMC is not set
 CONFIG_BOOTDELAY=1
 CONFIG_SPL=y
 CONFIG_CMD_BOOTZ=y