configs: stm32mp15: Enable OF_BOARD flag

Since commit 985503439762 ("fdt: Don't call board_fdt_blob_setup()
 without OF_BOARD") board_fdt_blob_setup() is no more called on
STM32MP platforms in trusted boot which hangs during boot process.

Enable OF_BOARD flag to fix this issue.

Signed-off-by: Patrice Chotard <patrice.chotard@foss.st.com>
Reviewed-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
diff --git a/arch/arm/mach-stm32mp/Kconfig b/arch/arm/mach-stm32mp/Kconfig
index 811c0fc..e48f98b 100644
--- a/arch/arm/mach-stm32mp/Kconfig
+++ b/arch/arm/mach-stm32mp/Kconfig
@@ -40,6 +40,7 @@
 	select CPU_V7A
 	select CPU_V7_HAS_NONSEC
 	select CPU_V7_HAS_VIRT
+	select OF_BOARD if TFABOOT
 	select OF_BOARD_SETUP
 	select PINCTRL_STM32
 	select STM32_RCC