rtc: sprd: Change to use devm_rtc_allocate_device()

This is a preparation patch, changing to use devm_rtc_allocate_device()
that can allow driver to set 'range_max' and 'range_min' for the RTC
device.

Signed-off-by: Baolin Wang <baolin.wang@linaro.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>