configs: stm32f746-disco: Rename stm32f746-disco_defconfig to stm32f746-disco_spl_defconfig

The current stm32f746-disco_defconfig file supports SPL, rename it to
stm32f746-disco_spl_defconfig to reflect the supported configuration.

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