misc: i2c_eeprom: implement different probe test eeprom offset

Because of this commit :
5ae84860 ("misc: i2c_eeprom: verify that the chip is functional at probe()")
at probe time, each eeprom is tested for read at offset 0.

The Atmel AT24MAC402 eeprom has different mapping. One i2c slave address is
used for the lower 0x80 bytes and another i2c slave address is used for the
upper 0x80 bytes. Because of this basically the i2c master sees 2 different
slaves. We need the upper bytes because we read the unique MAC address from
this EEPROM area.

However this implies that our slave address will return error on reads
from address 0x0 to 0x80.

To solve this, implemented an offset field inside platform data that is by
default 0 (as it is used now), but can be changed in the compatible table.

The probe function will now read at this offset and use it, instead of blindly
checking offset 0.

This will fix the regression noticed on these EEPROMs since the commit
abovementioned that introduces the probe failed issue.

Signed-off-by: Eugen Hristev <eugen.hristev@microchip.com>
Reviewed-by: Heiko Schocher <hs@denx.de>