rtc: isl12022: Get rid of unneeded private struct isl12022

First of all, the struct rtc_device pointer is kept in the managed
resources, no need to keep it outside (no users in the driver).

Second, replace private struct isl12022 with a regmap.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Link: https://lore.kernel.org/r/20230110140806.87432-2-andriy.shevchenko@linux.intel.com


Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>