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_dbm_soc1_defconfig b/configs/socfpga_dbm_soc1_defconfig
index baae2f2..15652a1 100644
--- a/configs/socfpga_dbm_soc1_defconfig
+++ b/configs/socfpga_dbm_soc1_defconfig
@@ -8,6 +8,7 @@
 CONFIG_SPL_TEXT_BASE=0xFFFF0000
 CONFIG_TARGET_SOCFPGA_DEVBOARDS_DBM_SOC1=y
 CONFIG_FIT=y
+CONFIG_TIMESTAMP=y
 CONFIG_USE_BOOTARGS=y
 CONFIG_BOOTARGS="console=ttyS0,115200"
 CONFIG_USE_BOOTCOMMAND=y