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/am335x_sl50_defconfig b/configs/am335x_sl50_defconfig
index aa1e196..b65f960 100644
--- a/configs/am335x_sl50_defconfig
+++ b/configs/am335x_sl50_defconfig
@@ -15,6 +15,7 @@
 CONFIG_SPL_FS_FAT=y
 CONFIG_SPL_LIBDISK_SUPPORT=y
 CONFIG_DISTRO_DEFAULTS=y
+CONFIG_TIMESTAMP=y
 CONFIG_SYS_EXTRA_OPTIONS="EMMC_BOOT"
 CONFIG_AUTOBOOT_KEYED=y
 CONFIG_AUTOBOOT_PROMPT="Press SPACE to abort autoboot in %d seconds\n"