stm32f4: fix MAINTAINERS file

When "scripts/get_maintainer.pl" parses "board/.../MAINTAINERS",
it uses the line containing board name as delimiter.
Without this line, the script happily mixes the lines from current
board MAINTAINERS file with lines from another file.

Fix it by adding a reasonable board name.

Tested by comparing output of:
	cat board/st/stm32f429-discovery/MAINTAINERS
	./scripts/get_maintainer.pl -f board/st/stm32f429-discovery

Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
To: u-boot@lists.denx.de
To: Kamil Lulko <rev13@wp.pl>
Cc: Tom Rini <trini@konsulko.com>