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_arria10_defconfig b/configs/socfpga_arria10_defconfig
index d6b7444..cd1c68a 100644
--- a/configs/socfpga_arria10_defconfig
+++ b/configs/socfpga_arria10_defconfig
@@ -12,6 +12,7 @@
 CONFIG_SPL_FS_FAT=y
 CONFIG_DISTRO_DEFAULTS=y
 CONFIG_FIT=y
+CONFIG_TIMESTAMP=y
 CONFIG_SPL_FIT=y
 # CONFIG_USE_SPL_FIT_GENERATOR is not set
 CONFIG_USE_BOOTARGS=y