powerpc: use time64_t in update_persistent_clock

update_persistent_clock() is deprecated because it suffers from overflow
in 2038 on 32-bit architectures. This changes powerpc to use the
update_persistent_clock64() replacement, and to pass down 64-bit
timestamps consistently.

This is now simpler, as we no longer have to worry about the offset
numbers in tm_year and tm_mon that are different between the Linux
conventions and RTAS.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>