rtc: ftrtc010: switch to devm_rtc_allocate_device

Switch to devm_rtc_allocate_device/rtc_register_device. This allow or
further improvement and simplifies ftrtc010_rtc_remove().

Acked-by: Hans Ulli Kroll <ulli.kroll@googlemail.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>