board: stm32: Fix stm32f746-disco boot

Since 'commit af2f44267 ("fdc spl: use different BOARD_INIT MACRO for spl and tpl")'
board stm32f746-disco can't boot.

The macro CONFIG_IS_ENABLED() can't evaluate the value of
CONFIG_SPL_BOARD_INIT as it was defined in include/configs/stm32f746-disco.h
without attributed value.

Moving CONFIG_SPL_BOARD_INIT from stm32f746-disco.h to mach-stm32/Kconfig
fixes this issue.

Fixes: af2f44267 ("fdc spl: use different BOARD_INIT MACRO for spl and tpl")
Signed-off-by: Patrice Chotard <patrice.chotard@st.com>
diff --git a/arch/arm/mach-stm32/Kconfig b/arch/arm/mach-stm32/Kconfig
index 5f7a2b5..f79b1a2 100644
--- a/arch/arm/mach-stm32/Kconfig
+++ b/arch/arm/mach-stm32/Kconfig
@@ -29,6 +29,7 @@
 	select STM32_SERIAL
 	select SUPPORT_SPL
 	select SPL
+	select SPL_BOARD_INIT
 	select SPL_CLK
 	select SPL_DM
 	select SPL_DM_SEQ_ALIAS