stm32mp1: migrate PREBOOT to Kconfig

Use Kconfig to activate CONFIG_PREBOOT (empty by default).

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
index efb9aab..01ff57c 100644
--- a/arch/arm/Kconfig
+++ b/arch/arm/Kconfig
@@ -1488,6 +1488,7 @@
 	imply CMD_DM
 	imply CMD_POWEROFF
 	imply ENV_VARS_UBOOT_RUNTIME_CONFIG
+	imply USE_PREBOOT
 	help
 	  Support for STM32MP SoC family developed by STMicroelectronics,
 	  MPUs based on ARM cortex A core