rtc: s3c: Do not remove const from rodata memory

All instances of struct s3c_rtc_data are in fact static const thus
put in rodata so we should not drop the const while getting the pointer
to them.

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>