dt-bindings: display: add rotation property to sitronix,st7789v

The sitronix-st7789v driver now considers the rotation property.
Add the property to the documentation.

Acked-by: Conor Dooley <conor.dooley@microchip.com>
Reviewed-by: Sebastian Reichel <sre@kernel.org>
Signed-off-by: Michael Riesch <michael.riesch@wolfvision.net>
Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20230718-feature-st7789v-v3-3-157d68fb63e2@wolfvision.net