Convert AT24 driver to use NVMEM subsystem instead of explicitly
creating a dedicated cdev. This way it becomes possible to access the
contenst of EEPROM via NVMEM API, which could be usefull for things
like MAC-addresses and such.

Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>