powerpc/spufs: use timespec64 for timestamps

The switch log prints the tv_sec portion of timespec as a 32-bit
number, while overflows in 2106. It also uses the timespec type,
which is safe on 64-bit architectures, but deprecated because
it causes overflows in 2038 elsewhere.

This changes it to timespec64 and printing a 64-bit number for
consistency.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Jeremy Kerr <jk@ozlabs.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>