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/powerpc/cpu/mpc85xx/Kconfig b/arch/powerpc/cpu/mpc85xx/Kconfig
index 4471754..c308447 100644
--- a/arch/powerpc/cpu/mpc85xx/Kconfig
+++ b/arch/powerpc/cpu/mpc85xx/Kconfig
@@ -381,6 +381,7 @@
 	imply CMD_PCI
 	imply CMD_REGINFO
 	imply FSL_SATA
+	imply TIMESTAMP
 
 config ARCH_P1011
 	bool
@@ -512,6 +513,7 @@
 	imply CMD_EEPROM
 	imply CMD_NAND
 	imply CMD_REGINFO
+	imply TIMESTAMP
 
 config ARCH_P2041
 	bool