rtc: dev: properly manage lifetime of dev and cdev in rtc device

struct rtc embeds both struct dev and struct cdev.  Unfortunately character
device structure may outlive the parent rtc structure unless we set it up
as parent of character device so that it will stay pinned until character
device is freed.

Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Reviewed-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>