dt-bindings: panel: Add missing .txt suffix

All other files in that directory have a .txt suffix, so add one for
consistency.
Signed-off-by: Thierry Reding <treding@nvidia.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20181203161310.15438-1-thierry.reding@gmail.com