dt-bindings: panel: rocktech,jh057n00900: Add myself as maintainer

I maintained the txt based bindings before yaml conversion and care
about the driver.
Signed-off-by: Guido Günther <agx@sigxcpu.org>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/9427a9c0e6aaf9fb375f7ecee6691ba491149d52.1597561897.git.agx@sigxcpu.org

Documentation/devicetree/bindings/display/panel/rocktech,jh057n00900.yaml

@@ -8,6 +8,7 @@ title: Rocktech JH057N00900 5.5" 720x1440 TFT LCD panel
 
 maintainers:
   - Ondrej Jirman <megi@xff.cz>
+  - Guido Gŭnther <agx@sigxcpu.org>
 
 description:
   Rocktech JH057N00900 is a 720x1440 TFT LCD panel