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/uniphier_v7_defconfig b/configs/uniphier_v7_defconfig
index 9089be0..87eb939 100644
--- a/configs/uniphier_v7_defconfig
+++ b/configs/uniphier_v7_defconfig
@@ -11,6 +11,7 @@
 CONFIG_SPL=y
 CONFIG_MICRO_SUPPORT_CARD=y
 CONFIG_SYS_LOAD_ADDR=0x85000000
+CONFIG_TIMESTAMP=y
 # CONFIG_ARCH_FIXUP_FDT_MEMORY is not set
 CONFIG_BOOTCOMMAND="run ${bootdev}script; run ${bootdev}boot"
 CONFIG_USE_PREBOOT=y