doc: blkmap: Add introduction and examples

Explain block maps by going through two common use-cases.

Signed-off-by: Tobias Waldekranz <tobias@waldekranz.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/MAINTAINERS b/MAINTAINERS
index ec8b149..4c17c6c 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -797,6 +797,7 @@
 M:	Tobias Waldekranz <tobias@waldekranz.com>
 S:	Maintained
 F:	cmd/blkmap.c
+F:	doc/usage/blkmap.rst
 F:	drivers/block/blkmap.c
 F:	include/blkmap.h
 F:	test/dm/blkmap.c