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_v8_defconfig b/configs/uniphier_v8_defconfig
index e684291..b9e2d9b 100644
--- a/configs/uniphier_v8_defconfig
+++ b/configs/uniphier_v8_defconfig
@@ -10,6 +10,7 @@
 CONFIG_ARCH_UNIPHIER_V8_MULTI=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