rtc: Allow rtc drivers to specify the tv_nsec value for ntp

ntp is currently hardwired to try and call the rtc set when wall clock
tv_nsec is 0.5 seconds. This historical behaviour works well with certain
PC RTCs, but is not universal to all rtc hardware.

Change how this works by introducing the driver specific concept of
set_offset_nsec, the delay between current wall clock time and the target
time to set (with a 0 tv_nsecs).

For x86-style CMOS set_offset_nsec should be -0.5 s which causes the last
second to be written 0.5 s after it has started.

For compat with the old rtc_set_ntp_time, the value is defaulted to
+ 0.5 s, which causes the next second to be written 0.5s before it starts,
as things were before this patch.

Testing shows many non-x86 RTCs would like set_offset_nsec ~= 0,
so ultimately each RTC driver should set the set_offset_nsec according
to its needs, and non x86 architectures should stop using
update_persistent_clock64 in order to access this feature.
Future patches will revise the drivers as needed.

Since CMOS and RTC now have very different handling they are split
into two dedicated code paths, sharing the support code, and ifdefs
are replaced with IS_ENABLED.

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: Jason Gunthorpe <jgunthorpe@obsidianresearch.com>
Signed-off-by: John Stultz <john.stultz@linaro.org>

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