[S390] introduce vdso on s390

Add a vdso to speed up gettimeofday and clock_getres/clock_gettime for
CLOCK_REALTIME/CLOCK_MONOTONIC.

Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
diff --git a/arch/s390/Kconfig b/arch/s390/Kconfig
index 8116a33..37bb373 100644
--- a/arch/s390/Kconfig
+++ b/arch/s390/Kconfig
@@ -43,6 +43,9 @@
 config GENERIC_TIME
 	def_bool y
 
+config GENERIC_TIME_VSYSCALL
+	def_bool y
+
 config GENERIC_CLOCKEVENTS
 	def_bool y