i2c: slave-eeprom: Make it possible to pre-load eeprom data

If the slave eeprom has a "firmware-name" in devicetree, then pre-load
the data in the eeprom with this file. Otherwise we init the eeprom with
0xFF.

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Signed-off-by: Björn Ardö <bjorn.ardo@axis.com>
[wsa: some cosmetic changes]
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>