rtc: pcf85063: do not register a RTC device if chip is not present

Add a sanity check to see if chip is present. If we can not communicate
with the chip there is no point in registering a RTC device.

Signed-off-by: Mirza Krak <mirza.krak@hostmobility.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>