configs: stm32f746-disco: Fix SPL boot

configs: stm32f746-disco: Fix SPL boot

Commit 'b4b9a00ed593 ("Convert CONFIG_SYS_SPL_ARGS_ADDR to Kconfig")'
replaces CONFIG_SYS_FDT_BASE by CONFIG_SYS_SPL_ARGS_ADDR.
As CONFIG_SYS_SPL_ARGS_ADDR enables additional code when enable, it
increases SPL size over the initial 0x8000 limit.
Increase the SPL size to 0x9000 to fix SPL boot.
Set SPL_SIZE_LIMIT to 0x9000 to avoid similar issue in the future.

Fixes 'b4b9a00ed593 ("Convert CONFIG_SYS_SPL_ARGS_ADDR to Kconfig")'

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>