rtc: isl1208: Introduce driver state struct

This driver has no state of its own, depending entirely on what is in
the generic rtc device.

Intoduce a state struct.  For now it only contains a pointer to the rtc
device struct, but future patches will add more data.

Cc: Alessandro Zummo <a.zummo@towertech.it>
Cc: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Trent Piepho <tpiepho@impinj.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>