configs: stm32f746-disco: Fix CONFIG_SYS_SPL_ARGS_ADDR

STM32F746 embeds 1 MB of internal flash [0x08000000-0x080fffff],
fix CONFIG_SYS_SPL_ARGS_ADDR accordingly
It solves hard fault when jumping from SPL to U-Boot.

Signed-off-by: Patrice Chotard <patrice.chotard@foss.st.com>
Reviewed-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Tested-by: Waldemar Brodkorb <wbx@openadk.org>