stm32mp1: add command poweroff

Activate the command poweroff by default for STM32MP1:
- with PCSI from TF-A for trusted boot
- with PMIC sysreset request for basic boot (SYSRESET_POWER)

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
index 33628b8..28d4d35 100644
--- a/arch/arm/Kconfig
+++ b/arch/arm/Kconfig
@@ -1403,6 +1403,7 @@
 	select SYSRESET
 	select SYS_THUMB_BUILD
 	imply CMD_DM
+	imply CMD_POWEROFF
 	imply ENV_VARS_UBOOT_RUNTIME_CONFIG
 	help
 	  Support for STM32MP SoC family developed by STMicroelectronics,