doc: boards: amlogic: update for Radxa Zero

Add documentation bits for the Radxa Zero

Signed-off-by: Christian Hewitt <christianshewitt@gmail.com>
[narmstrong: updated u200 MAINTAINERS]
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
diff --git a/board/amlogic/u200/MAINTAINERS b/board/amlogic/u200/MAINTAINERS
index 73c73b1..a259d12 100644
--- a/board/amlogic/u200/MAINTAINERS
+++ b/board/amlogic/u200/MAINTAINERS
@@ -7,3 +7,4 @@
 F:	configs/bananapi-m5_defconfig
 F:	configs/radxa-zero_defconfig
 F:	doc/board/amlogic/u200.rst
+F:	doc/board/amlogic/radxa-zero.rst