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/imx28_xea_defconfig b/configs/imx28_xea_defconfig
index 5192dd2..3404f39 100644
--- a/configs/imx28_xea_defconfig
+++ b/configs/imx28_xea_defconfig
@@ -23,6 +23,7 @@
 CONFIG_SPL_PAYLOAD="u-boot.img"
 CONFIG_SYS_LOAD_ADDR=0x42000000
 CONFIG_FIT=y
+CONFIG_TIMESTAMP=y
 CONFIG_OF_BOARD_SETUP=y
 CONFIG_USE_BOOTARGS=y
 CONFIG_BOOTARGS="console=ttyAMA0,115200n8"