powerpc/rtas: Make timestamp related code y2038-safe

While we are here, let us make timestamp related code y2038-safe.

Suggested-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Hari Bathini <hbathini@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>