time: Add a timeval with a 32-bit tv_sec and tv_usec

On y2038 safe 32-bit systems the Linux kernel expects itimerval to
use a 32-bit time_t, even though the other time_t's are 64-bit. To
address this let's add a __timeval32 struct to be used internally.

Reviewed-by: Lukasz Majewski <lukma@denx.de>
Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>