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>

diff --cc arch/powerpc/include/asm/rtas.h
Simple merge

diff --cc arch/powerpc/kernel/nvram_64.c
Simple merge

diff --cc arch/powerpc/platforms/pseries/nvram.c
Simple merge