dt-bindings: display: fix panel warnings

Fix following type af warnings in the panel bindings:

Warning (unit_address_vs_reg): /example-0/dsi/panel: node has a reg or ranges property, but no unit name
Warning (unit_address_vs_reg): /example-0/dsi@ff450000: node has a unit name, but no reg property

Removing the "@xxx" from the node name fixed first warning.
Adding a missing reg property fixed the second warning

v2:
  - renamed mdss_dsi to dsi in panel-simple-dsi.yaml (Rob)

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Benjamin Gaignard <benjamin.gaignard@st.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Cc: Thierry Reding <thierry.reding@gmail.com>
Cc: Linus Walleij <linus.walleij@linaro.org>
Cc: Rob Herring <robh@kernel.org>
Cc: Heiko Stuebner <heiko.stuebner@theobroma-systems.com>
Cc: Maxime Ripard <mripard@kernel.org>
Cc: Benjamin Gaignard <benjamin.gaignard@st.com>
Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200308115017.18563-2-sam@ravnborg.org