i2c: Move at24 to drivers/misc/eeprom

As drivers/i2c/chips is going to go away, move the driver to
drivers/misc/eeprom. Other eeprom drivers may be moved here later, too.
Update Kconfig text to specify this driver as I2C.

Signed-off-by: Wolfram Sang <w.sang@pengutronix.de>
Signed-off-by: Jean Delvare <khali@linux-fr.org>