dt-bindings: display/panel: jh057n00900: Document power supply properties

Document the vcc-supply and iovcc-supply properties of the Rocktech
jh057n00900 panel.

Signed-off-by: Guido Günther <agx@sigxcpu.org>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/ec5ee11786036dc8826a3f8555cbb1dce4dbca25.1561542477.git.agx@sigxcpu.org