board: stm32f746-disco: Fix dram_init() in none SPL config

Replace CONFIG_SUPPORT_SPL by CONFIG_SPL_BUILD to allow
dram_init() execution when using none SPL defconfig
(stm32f746-disco_defconfig).

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