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_is1_defconfig b/configs/socfpga_is1_defconfig
index 7b27bcc..2124a08 100644
--- a/configs/socfpga_is1_defconfig
+++ b/configs/socfpga_is1_defconfig
@@ -11,6 +11,7 @@
 CONFIG_TARGET_SOCFPGA_IS1=y
 CONFIG_DISTRO_DEFAULTS=y
 CONFIG_FIT=y
+CONFIG_TIMESTAMP=y
 CONFIG_USE_BOOTARGS=y
 CONFIG_BOOTARGS="console=ttyS0,115200"
 # CONFIG_USE_BOOTCOMMAND is not set