configs: stm32f746-disco: use CONFIG_DEFAULT_DEVICE_TREE as fdtfile

As stm32f46-disco, stm32f769-disco and stm32746g-eval are very similar
except their respective device tree file. These 3 boards uses the same
TARGET_STM32F746_DISCO flag (so same include/configs/stm32f746-disco.h
and same board file board/st/stm32f746-disco/stm32f746-disco.c)

To be able to compile these 3 boards, replace the hard-coded device-tree
name in include/configs/stm32f746-disco.h by CONFIG_DEFAULT_DEVICE_TREE
which is set in each board defconfig file with the correct value.

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