dm: mmc: Move the device list into a separate file

At present the MMC subsystem maintains its own list of MMC devices. This
cannot work with driver model, which needs to maintain this itself. Move the
list code into a separate 'legacy' file. The core MMC code remains, and will
be shared with the driver-model implementation.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/drivers/mmc/mmc_write.c b/drivers/mmc/mmc_write.c
index f4d42aa..bd07b20 100644
--- a/drivers/mmc/mmc_write.c
+++ b/drivers/mmc/mmc_write.c
@@ -122,9 +122,9 @@
 	struct mmc_data data;
 	int timeout = 1000;
 
-	if ((start + blkcnt) > mmc->block_dev.lba) {
+	if ((start + blkcnt) > mmc_get_blk_desc(mmc)->lba) {
 		printf("MMC: block number 0x" LBAF " exceeds max(0x" LBAF ")\n",
-		       start + blkcnt, mmc->block_dev.lba);
+		       start + blkcnt, mmc_get_blk_desc(mmc)->lba);
 		return 0;
 	}