dt-bindings: display: Convert stm32 display bindings to json-schema

Convert the STM32 display binding to DT schema format using json-schema.
Split the original bindings in two yaml files:
- one for display controller (ltdc)
- one for DSI controller

Signed-off-by: Benjamin Gaignard <benjamin.gaignard@st.com>
[robh: drop maxItems from phy-dsi-supply]
Signed-off-by: Rob Herring <robh@kernel.org>