Convert CONFIG_TIMESTAMP to Kconfig

This converts the following to Kconfig:
   CONFIG_TIMESTAMP

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/Kconfig b/arch/Kconfig
index ee32e83..f2deec1 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -256,6 +256,7 @@
 	imply ACPIGEN if !QEMU
 	imply SYSINFO if GENERATE_SMBIOS_TABLE
 	imply SYSINFO_SMBIOS if GENERATE_SMBIOS_TABLE
+	imply TIMESTAMP
 
 	# Thing to enable for when SPL/TPL are enabled: SPL
 	imply SPL_DM