dt-bindings: display: bridge: renesas,dsi-csi2-tx: Align panel example with ili9881c binding

Update the panel example in this DT schema to match requirements in binding
display/panel/ilitek,ili9881c.yaml . This fixes the following schema check
warnings:

"
/tmp/dtx/Documentation/devicetree/bindings/display/bridge/renesas,dsi-csi2-tx.example.dtb: panel@0 (raspberrypi,dsi-7inch): compatible: ['raspberrypi,dsi-7inch'] is too short
from schema $id: http://devicetree.org/schemas/display/panel/ilitek,ili9881c.yaml
/tmp/dtx/Documentation/devicetree/bindings/display/bridge/renesas,dsi-csi2-tx.example.dtb: panel@0 (raspberrypi,dsi-7inch): 'power-supply' is a required property
from schema $id: http://devicetree.org/schemas/display/panel/ilitek,ili9881c.yaml
"

Fixes: c376a6943741 ("dt-bindings: display: bridge: renesas,dsi-csi2-tx: Allow panel@ subnode")
Signed-off-by: Marek Vasut <marek.vasut+renesas@mailbox.org>
Reviewed-by: Neil Armstrong <neil.armstrong@linaro.org>
Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>
Link: https://patch.msgid.link/20251029200519.214548-1-marek.vasut+renesas@mailbox.org