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>
diff --git a/board/st/stm32f746-disco/MAINTAINERS b/board/st/stm32f746-disco/MAINTAINERS
index 3bbb513..06d366e 100644
--- a/board/st/stm32f746-disco/MAINTAINERS
+++ b/board/st/stm32f746-disco/MAINTAINERS
@@ -4,4 +4,5 @@
 F:	board/st/stm32f746-disco
 F:	include/configs/stm32f746-disco.h
 F:	configs/stm32f746-disco_defconfig
+F:	configs/stm32f746-disco_spl_defconfig
 F:	configs/stm32f769-disco_defconfig