rtc: gemini: add return value validation

Function devm_ioremap() will return a NULL pointer if it fails to remap
IO address, and its return value should be validated before it is used.
However, in function gemini_rtc_probe(), its return value is not
checked. This may result in bad memory access bugs on future access,
e.g. calling the function gemini_rtc_read_time().

Signed-off-by: Pan Bian <bianpan2016@163.com>
Acked-by: Hans Ulli Kroll <ulli.kroll@googlemail.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>