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/socfpga_de0_nano_soc_defconfig b/configs/socfpga_de0_nano_soc_defconfig
index 826b14e..09b51e1 100644
--- a/configs/socfpga_de0_nano_soc_defconfig
+++ b/configs/socfpga_de0_nano_soc_defconfig
@@ -9,6 +9,7 @@
 CONFIG_TARGET_SOCFPGA_TERASIC_DE0_NANO=y
 CONFIG_DISTRO_DEFAULTS=y
 CONFIG_FIT=y
+CONFIG_TIMESTAMP=y
 # CONFIG_USE_BOOTCOMMAND is not set
 CONFIG_DEFAULT_FDT_FILE="socfpga_cyclone5_de0_nano_soc.dtb"
 CONFIG_SYS_CONSOLE_IS_IN_ENV=y