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/stmark2_defconfig b/configs/stmark2_defconfig
index 53510ea..6ccd667 100644
--- a/configs/stmark2_defconfig
+++ b/configs/stmark2_defconfig
@@ -7,6 +7,7 @@
 CONFIG_DEFAULT_DEVICE_TREE="stmark2"
 CONFIG_TARGET_STMARK2=y
 CONFIG_SYS_LOAD_ADDR=0x40010000
+CONFIG_TIMESTAMP=y
 CONFIG_SYS_EXTRA_OPTIONS="CF_SBF,SYS_SERIAL_BOOT,SYS_INPUT_CLKSRC=30000000"
 CONFIG_USE_BOOTARGS=y
 CONFIG_BOOTARGS="console=ttyS0,115200 root=/dev/ram0 rw rootfstype=ramfs rdinit=/bin/init devtmpfs.mount=1"