imx8m: beacon: Update MAINTAINER file to include beacon rst files

With variou README files migrated to rst, add them to the
MAINTAINER file for Beacon.

Signed-off-by: Adam Ford <aford173@gmail.com>
diff --git a/board/beacon/imx8mm/MAINTAINERS b/board/beacon/imx8mm/MAINTAINERS
index e887db2..d48ba86 100644
--- a/board/beacon/imx8mm/MAINTAINERS
+++ b/board/beacon/imx8mm/MAINTAINERS
@@ -5,3 +5,4 @@
 F:	board/beacon/imx8mm/
 F:	include/configs/imx8mm_beacon.h
 F:	configs/imx8mm_beacon_defconfig
+F:	doc/board/beacon/