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_evm_spiboot_defconfig b/configs/am335x_evm_spiboot_defconfig
index a856457..f8acb7e 100644
--- a/configs/am335x_evm_spiboot_defconfig
+++ b/configs/am335x_evm_spiboot_defconfig
@@ -11,6 +11,7 @@
 CONFIG_SPL_SPI_FLASH_SUPPORT=y
 CONFIG_SPL_SPI=y
 CONFIG_DISTRO_DEFAULTS=y
+CONFIG_TIMESTAMP=y
 CONFIG_SPL_LOAD_FIT=y
 CONFIG_OF_BOARD_SETUP=y
 CONFIG_BOOTCOMMAND="run findfdt; run init_console; run finduuid; run distro_bootcmd"