board: amlogic: add board doc files to MAINTAINERS

Add the recently added reStructuredText board documentation in the
appropriate MAINTAINERS files.

Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
diff --git a/board/amlogic/s400/MAINTAINERS b/board/amlogic/s400/MAINTAINERS
index da90cf9..fb46b1b 100644
--- a/board/amlogic/s400/MAINTAINERS
+++ b/board/amlogic/s400/MAINTAINERS
@@ -5,3 +5,4 @@
 F:	board/amlogic/s400/
 F:	include/configs/s400.h
 F:	configs/s400_defconfig
+F:	doc/board/amlogic/s400.rst