time64.h: Consolidated PSEC_PER_SEC definition

We have currently three users of the PSEC_PER_SEC each of them defining it
individually. Instead, move it to time64.h to be available for everyone.

There is a new user coming with the same constant in use. It will also
make its life easier.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: David S. Miller <davem@davemloft.net>