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/sei510/MAINTAINERS b/board/amlogic/sei510/MAINTAINERS
index 8dc655e..c01c1d6 100644
--- a/board/amlogic/sei510/MAINTAINERS
+++ b/board/amlogic/sei510/MAINTAINERS
@@ -5,3 +5,4 @@
 F:	board/amlogic/sei510/
 F:	configs/sei510_defconfig
 F:	include/configs/sei510.h
+F:	doc/board/amlogic/sei510.rst