dts: correct compatible name for I2C EEPROM devices

Zephyr has been using "i2c,eeprom" for the compatible property.  This is
inconsistent with the property documentation which expects
"manufacturer,model" to be used.

The Linux standard compatible name for I2C EEPROMs is "atmel,at24".  The
standard for SPI EEPROMS is "atmel,at25".  We don't have support for the
latter, but change the name for the former.

Signed-off-by: Peter A. Bigot <pab@pabigot.com>