Add board MAINTAINERS files

We have switched to Kconfig and the boards.cfg file is going to
be removed. We have to retrieve the board status and maintainers
information from it.

The MAINTAINERS format as in Linux Kernel would be nice
because we can crib the scripts/get_maintainer.pl script.

After some discussion, we chose to put a MAINTAINERS file under each
board directory, not the top-level one because we want to collect
relevant information for a board into a single place.

TODO:
Modify get_maintainer.pl to scan multiple MAINTAINERS files.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Suggested-by: Tom Rini <trini@ti.com>
Acked-by: Simon Glass <sjg@chromium.org>