timekeeping: Make fast accessors return 0 before timekeeping is initialized

printk timestamps will be extended to include mono and boot time by using
the fast timekeeping accessors ktime_get_mono|boot_fast_ns().  The
functions can return garbage before timekeeping is initialized resulting in
garbage timestamps.

Initialize the fast timekeepers with dummy clocks which guarantee a 0
readout up to timekeeping_init().

Suggested-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Prarit Bhargava <prarit@redhat.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Stephen Boyd <sboyd@codeaurora.org>
Cc: John Stultz <john.stultz@linaro.org>
Link: http://lkml.kernel.org/r/1503922914-10660-2-git-send-email-prarit@redhat.com