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_sr1500_defconfig b/configs/socfpga_sr1500_defconfig
index c400cbc..a7a48a0 100644
--- a/configs/socfpga_sr1500_defconfig
+++ b/configs/socfpga_sr1500_defconfig
@@ -14,6 +14,7 @@
 CONFIG_ENV_OFFSET_REDUND=0xF0000
 CONFIG_DISTRO_DEFAULTS=y
 CONFIG_FIT=y
+CONFIG_TIMESTAMP=y
 # CONFIG_USE_BOOTCOMMAND is not set
 CONFIG_DEFAULT_FDT_FILE="socfpga_cyclone5_sr1500.dtb"
 CONFIG_SYS_CONSOLE_IS_IN_ENV=y