configs: stm32f746-disco: Add stm32f746-disco_defconfig

Add stm32f746-disco_defconfig for stm32f746 discovery board to
build U-Boot proper.

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