Convert CONFIG_TIMESTAMP to Kconfig

This converts the following to Kconfig:
   CONFIG_TIMESTAMP

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/configs/cm_t335_defconfig b/configs/cm_t335_defconfig
index 0600bf5..7a65683 100644
--- a/configs/cm_t335_defconfig
+++ b/configs/cm_t335_defconfig
@@ -14,6 +14,7 @@
 CONFIG_SPL_FS_FAT=y
 CONFIG_SPL_LIBDISK_SUPPORT=y
 CONFIG_DISTRO_DEFAULTS=y
+CONFIG_TIMESTAMP=y
 # CONFIG_USE_BOOTCOMMAND is not set
 CONFIG_SYS_CONSOLE_INFO_QUIET=y
 CONFIG_SPL_FS_EXT4=y