tools: timer: add rtctest_setdate

This tool allow to set directly the time and date to a RTC device.

Unlike other tools isn't doens't use "struct timeval" or "time_t"
so it is safe for 32bits platforms when testing for y2038/2106 bug.

Signed-off-by: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>