Convert CONFIG_TIMESTAMP to Kconfig

This converts the following to Kconfig:
   CONFIG_TIMESTAMP

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
index 8fc1897..6b11c3a 100644
--- a/arch/arm/Kconfig
+++ b/arch/arm/Kconfig
@@ -1869,6 +1869,7 @@
 	imply OF_LIBFDT_OVERLAY
 	imply ENV_VARS_UBOOT_RUNTIME_CONFIG
 	imply USE_PREBOOT
+	imply TIMESTAMP
 	help
 	  Support for STM32MP SoC family developed by STMicroelectronics,
 	  MPUs based on ARM cortex A core