i2c: slave-eeprom: Add read only mode

Add read-only versions of all EEPROMs. These versions are read-only
on the i2c side, but can be written from the sysfs side.

Signed-off-by: Björn Ardö <bjorn.ardo@axis.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>