ARM: dts: stm32: Migrate U-boot nodes to U-boot DT files for stm32f7

In order to prepare and ease future DT synchronization with kernel
DT, migrate all U-boot specific nodes/properties/addons to
U-boot DT files.

Migrate also DT nodes which are not yet available on kernel DT side
as ethernet, ltdc and qspi nodes.

Fix ethernet_mii pins and add missing qspi_pins for stm32746g-eval

Signed-off-by: Patrice Chotard <patrice.chotard@st.com>