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_uart_defconfig b/configs/am335x_hs_evm_uart_defconfig
index bca1889..ecb0526 100644
--- a/configs/am335x_hs_evm_uart_defconfig
+++ b/configs/am335x_hs_evm_uart_defconfig
@@ -11,6 +11,7 @@
 # CONFIG_SPL_FS_FAT is not set
 # CONFIG_SPL_LIBDISK_SUPPORT is not set
 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