ARM: dts: stm32: Add power-supply for DSI panel on stm32f469-disco

Add a fixed regulator and use it as power supply for DSI panel.

Fixes: 18c88662 ("ARM: dts: stm32: Add display support on stm32f469-disco")

Signed-off-by: Benjamin Gaignard <benjamin.gaignard@st.com>
Signed-off-by: Alexandre Torgue <alexandre.torgue@st.com>