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