ARM: meson: documentation: add bindings documentation

Add vendor prefixes and basic documentation for MesonX SoCs bindings
Signed-off-by: Carlo Caione <carlo@caione.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>

Documentation/devicetree/bindings/arm/amlogic.txt

+Amlogic MesonX device tree bindings
+-------------------------------------------
+
+Boards with the Amlogic Meson6 SoC shall have the following properties:
+
+Required root node property:
+
+compatible = "amlogic,meson6";

Documentation/devicetree/bindings/arm/geniatech.txt

+Geniatech platforms device tree bindings
+-------------------------------------------
+
+Geniatech ATV1200
+    - compatible = "geniatech,atv1200"

Documentation/devicetree/bindings/vendor-prefixes.txt

@@ -14,6 +14,7 @@ allwinner	Allwinner Technology Co., Ltd.
 altr	Altera Corp.
 amcc	Applied Micro Circuits Corporation (APM, formally AMCC)
 amd	Advanced Micro Devices (AMD), Inc.
+amlogic	Amlogic, Inc.
 ams	AMS AG
 amstaos	AMS-Taos Inc.
 apm	Applied Micro Circuits Corporation (APM)
@@ -53,6 +54,7 @@ excito	Excito
 fsl	Freescale Semiconductor
 GEFanuc	GE Fanuc Intelligent Platforms Embedded Systems, Inc.
 gef	GE Fanuc Intelligent Platforms Embedded Systems, Inc.
+geniatech	Geniatech, Inc.
 globalscale	Globalscale Technologies, Inc.
 gmt	Global Mixed-mode Technology, Inc.
 google	Google, Inc.