stm32mp1: configs: activate CONFIG_SILENT_CONSOLE

Allow to disable console with environment variable 'silent':
> env set silent 1; env save
Signed-off-by: NPatrick Delaunay <patrick.delaunay@st.com>

arch/arm/mach-stm32mp/Kconfig

@@ -47,6 +47,7 @@ config TARGET_STM32MP1
 	imply BOOTSTAGE
 	imply CMD_BOOTCOUNT
 	imply CMD_BOOTSTAGE
+	imply SILENT_CONSOLE
 	imply SYSRESET_PSCI if STM32MP1_TRUSTED
 	imply SYSRESET_SYSCON if !STM32MP1_TRUSTED
 	help