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/imgtec_xilfpga_defconfig b/configs/imgtec_xilfpga_defconfig
index 3ded3a0..576d68c 100644
--- a/configs/imgtec_xilfpga_defconfig
+++ b/configs/imgtec_xilfpga_defconfig
@@ -10,6 +10,7 @@
 # CONFIG_MIPS_BOOT_ENV_LEGACY is not set
 CONFIG_MIPS_BOOT_FDT=y
 CONFIG_SYS_LOAD_ADDR=0x80500000
+CONFIG_TIMESTAMP=y
 CONFIG_BOOTDELAY=5
 # CONFIG_DISPLAY_BOARDINFO is not set
 CONFIG_HUSH_PARSER=y