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/k2hk_hs_evm_defconfig b/configs/k2hk_hs_evm_defconfig
index c3ef1a7..e1b98a6 100644
--- a/configs/k2hk_hs_evm_defconfig
+++ b/configs/k2hk_hs_evm_defconfig
@@ -14,6 +14,7 @@
 CONFIG_ENV_OFFSET=0x100000
 CONFIG_DEFAULT_DEVICE_TREE="keystone-k2hk-evm"
 CONFIG_DISTRO_DEFAULTS=y
+CONFIG_TIMESTAMP=y
 CONFIG_FIT_IMAGE_POST_PROCESS=y
 CONFIG_OF_BOARD_SETUP=y
 CONFIG_BOOTCOMMAND="run run_mon_hs; run init_${boot}; run get_fit_${boot}; bootm ${addr_fit}#${name_fdt}"