This patch adds an interface by which other kernel code can read/write
detected EEPROM.

The platform code registers a 'setup' callback with the
at24_platform_data.  When the at24 driver detects an EEPROM, it fills
out the read and write functions of at24_iface and calls the setup
callback.  The platform code can then use the read/write functions in
the at24_iface struct for reading and writing the EEPROM.

Original idea, review and updates by David Brownell <david-b@pacbell.net>
Signed-off-by: Kevin Hilman <khilman@deeprootsystems.com>