timekeeping: Add more coarse clocktai/boottime interfaces

The set of APIs we provide has a few holes for coarse times, e.g. we
provide ktime_get_coarse_boottime() and ktime_get_boottime_ts64(),
but not the combination of the two.

This adds four new functions:

ktime_get_coarse_boottime_ts64()
ktime_get_boottime_seconds()
ktime_get_coarse_clocktai_ts64()
ktime_get_clocktai_seconds()

to fill in some of the missing pieces. I have missed only the
ktime_get_boottime_seconds() accessor in a few occasions in
the past, but it seems better to just provide all four together,
as there is very little cost to having them.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Stephen Boyd <sboyd@kernel.org>
Cc: y2038@lists.linaro.org
Cc: John Stultz <john.stultz@linaro.org>
Link: https://lkml.kernel.org/r/20180427134016.2525989-6-arnd@arndb.de