dt-bindings: display: panel: Add bindings for the Sitronix ST7789V panel

The Sitronix ST7789V is an LCD panel controller, controlled over SPI, that
can drive 18-bits 240x320 LCD displays.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Thierry Reding <treding@nvidia.com>