bootstd: mmc: Add a bootdev driver

Add a bootdev driver for MMC. It mostly just calls the bootdev helper
function.

Add a function to obtain the block device for an MMC controller.

Fix up the comment for mmc_get_blk_desc() while we are here.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/MAINTAINERS b/MAINTAINERS
index d9125fc..239fa2e 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -702,6 +702,7 @@
 F:	boot/bootstd.c
 F:	cmd/bootdev.c
 F:	cmd/bootflow.c
+F:	drivers/mmc/mmc_bootdev.c
 F:	include/bootdev.h
 F:	include/bootflow.h
 F:	include/bootmeth.h