drivers: eeprom: reduce priority of at2x initialization

The default priority for I2C controller initialization is POST_KERNEL
60 (SPI 70), while the default priority for device configuration is
POST_KERNEL 50.  Thus the EEPROM is being initialized before its
controller.  While for this driver that wouldn't be an issue recent
changes mean the device lookup returns NULL before the device is
initialized.

Change the AT2X priority to 75 so it falls between the I2C and SPI
drivers and the default ethernet priority (80), since some ethernet
controllers may store the MAC address in EEPROM.

Signed-off-by: Peter Bigot <peter.bigot@nordicsemi.no>