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_hs_evm_defconfig b/configs/am335x_hs_evm_defconfig
index 21ef885..993fbbc 100644
--- a/configs/am335x_hs_evm_defconfig
+++ b/configs/am335x_hs_evm_defconfig
@@ -8,6 +8,7 @@
 CONFIG_AM33XX=y
 CONFIG_SPL=y
 CONFIG_DISTRO_DEFAULTS=y
+CONFIG_TIMESTAMP=y
 CONFIG_FIT_IMAGE_POST_PROCESS=y
 CONFIG_SPL_LOAD_FIT=y
 CONFIG_SPL_FIT_IMAGE_POST_PROCESS=y