rtc: sunxi: constify the data_year_param structure

The data_year_param struct is never modified, so lets constify it.
This permit to remove cast since of_device_id is const also.

Signed-off-by: LABBE Corentin <clabbe.montjoie@gmail.com>
Acked-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>