eeprom: at24: move platform data processing into a separate routine

This driver can receive its device data from different sources
depending on the system. Move the entire code processing platform data,
device tree and acpi into a separate function.

Signed-off-by: Bartosz Golaszewski <brgl@bgdev.pl>
Tested-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>