rtc: st-lpc: make it robust against y2038/2106 bug

Make driver use u64 variables and functions to be sure that
it will support dates after year 2038.

Signed-off-by: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Acked-by: Patrice Chotard <patrice.chotard@st.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>