ARM: dts: stm32: convert stm23f7 boards to OF_UPSTREAM

ARM: dts: stm32: convert stm23f7 boards to OF_UPSTREAM

Enable OF_UPSTREAM flag for STM32F7 platforms.

Use upstream device tree for DSI and LTDC nodes,
As now in upstream DT, in panel@0 node, power-supply property is
present, which is a fixed-regulator, add DM_REGULATOR_FIXED flag
for stm32f769-disco boards.

Set also DEFAULT_FDT_FILE in defconfigs and use it in stm32f746-disco.h
to indicate which FDT file to load (All STM32F7 boards are using this
file).

If something is missing, it must be added in upstream device tree
in linux kernel ("px_clk" for DSI by example).

Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Signed-off-by: Patrice Chotard <patrice.chotard@foss.st.com>
Reviewed-by: Patrick Delaunay <patrick.delaunay@foss.st.com>