configs: stm32746g-eval: Add stm32746g-eval_defconfig

Add stm32746g-eval_defconfig for stm32746g evaluation board to
build U-Boot proper.

Full board description can be found here :
https://www.st.com/en/evaluation-tools/stm32746g-eval.html



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