rtc: isl12026: new driver.

The ISL12026 is a combination RTC and EEPROM device with I2C
interface.  The standard RTC driver interface is provided.  The EEPROM
is accessed via the NVMEM interface.

Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: David Daney <david.daney@cavium.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>