Include the initial support for external EEPROMs.

Include the initial support for external EEPROMs.

The AR9100 at least doesn't have an external serial EEPROM
attached to the MAC; it instead stores the calibration data
in the normal system flash.

I believe earlier parts can do something similar but I haven't
experienced it first-hand.

This commit introduces an eepromdata pointer into the API but
doesn't at all commit to using it. A future commit will
include the glue needed to allow the AR9100 support code
to use this data pointer as the EEPROM.