timekeeping: Add missing _ns functions for coarse accessors

This further unifies the accessors for the fast and coarse functions, so
that the same types of functions are available for each. There was also
a bit of confusion with the documentation, which prior advertised a
function that has never existed. Finally, the vanilla ktime_get_coarse()
was omitted from the API originally, so this fills this oversight.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lkml.kernel.org/r/20190621203249.3909-3-Jason@zx2c4.com