time: Move old timekeeping interfaces to timekeeping32.h

The interfaces based on 'struct timespec' and 'unsigned long' seconds
are no longer recommended for new code, and we are trying to migrate to
ktime_t based interfaces and other y2038-safe variants.

This moves all the legacy interfaces from linux/timekeeping.h into a
new timekeeping32.h to better document this.

Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Miroslav Lichvar <mlichvar@redhat.com>
Cc: Richard Cochran <richardcochran@gmail.com>
Cc: Prarit Bhargava <prarit@redhat.com>
Cc: Stephen Boyd <stephen.boyd@linaro.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: John Stultz <john.stultz@linaro.org>