eeprom: at25: extend driver to plug into the NVMEM framework

Add a regmap for accessing the EEPROM, and then use that with the
NVMEM framework. Enable backwards compatibility in the NVMEM config,
so that the 'eeprom' file in sys is provided by the framework.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Acked-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>