dt-bindings: display: stm32: add a 2nd endpoint

ltdc can have up to 2 endpoints:
 - dpi external gpios: for rgb panels or external bridge ICs.
 - dpi internal ios: connected internally to dsi.

Note: Refer to the reference manual to know if the dsi is
present on your device.
Signed-off-by: Philippe Cornu <philippe.cornu@st.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/1509018489-19641-2-git-send-email-philippe.cornu@st.com

Documentation/devicetree/bindings/display/st,stm32-ltdc.txt

@@ -10,7 +10,11 @@
       - "lcd" for the clock feeding the output pixel clock & IP clock.
   - resets: reset to be used by the device (defined by use of RCC macro).
   Required nodes:
-    - Video port for RGB output.
+  - Video port for DPI RGB output: ltdc has one video port with up to 2
+    endpoints:
+      - for external dpi rgb panel or bridge, using gpios.
+      - for internal dpi input of the MIPI DSI host controller.
+      Note: These 2 endpoints cannot be activated simultaneously.
 
 * STMicroelectronics STM32 DSI controller specific extensions to Synopsys
   DesignWare MIPI DSI host controller