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_mcvevk_defconfig b/configs/socfpga_mcvevk_defconfig
index 2a2899d..d31c3f5 100644
--- a/configs/socfpga_mcvevk_defconfig
+++ b/configs/socfpga_mcvevk_defconfig
@@ -9,6 +9,7 @@
 CONFIG_TARGET_SOCFPGA_ARIES_MCVEVK=y
 CONFIG_DISTRO_DEFAULTS=y
 CONFIG_FIT=y
+CONFIG_TIMESTAMP=y
 CONFIG_USE_BOOTARGS=y
 CONFIG_BOOTARGS="console=ttyS0,115200"
 CONFIG_BOOTCOMMAND="run mmc_mmc"