clocksource: Provide clocksource_arch_init()

Architectures have extra archdata in the clocksource, e.g. for VDSO
support. There are no sanity checks or general initializations for this
available. Add support for that.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Andy Lutomirski <luto@kernel.org>
Acked-by: John Stultz <john.stultz@linaro.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Matt Rickard <matt@softrans.com.au>
Cc: Stephen Boyd <sboyd@kernel.org>
Cc: Florian Weimer <fweimer@redhat.com>
Cc: "K. Y. Srinivasan" <kys@microsoft.com>
Cc: Vitaly Kuznetsov <vkuznets@redhat.com>
Cc: devel@linuxdriverproject.org
Cc: virtualization@lists.linux-foundation.org
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Juergen Gross <jgross@suse.com>
Link: https://lkml.kernel.org/r/20180917130706.973042587@linutronix.de

diff --git a/include/linux/clocksource.h b/include/linux/clocksource.h
index 3089189..6e6b86f 100644
--- a/include/linux/clocksource.h
+++ b/include/linux/clocksource.h
@@ -241,6 +241,11 @@ static inline void __clocksource_update_freq_khz(struct clocksource *cs, u32 khz
 	__clocksource_update_freq_scale(cs, 1000, khz);
 }
 
+#ifdef CONFIG_ARCH_CLOCKSOURCE_INIT
+extern void clocksource_arch_init(struct clocksource *cs);
+#else
+static inline void clocksource_arch_init(struct clocksource *cs) { }
+#endif
 
 extern int timekeeping_notify(struct clocksource *clock);