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_defconfig b/configs/am335x_evm_defconfig
index 4dc5d0f..98dadf8 100644
--- a/configs/am335x_evm_defconfig
+++ b/configs/am335x_evm_defconfig
@@ -6,6 +6,7 @@
 CONFIG_AM33XX=y
 CONFIG_SPL=y
 CONFIG_DISTRO_DEFAULTS=y
+CONFIG_TIMESTAMP=y
 CONFIG_SPL_LOAD_FIT=y
 # CONFIG_USE_SPL_FIT_GENERATOR is not set
 CONFIG_OF_BOARD_SETUP=y