e1000: Export core EEPROM access functions for SPI support

A followup patch will be adding a configurable feature to enable
programming of E1000 EEPROMs from the command line or via the generic
U-Boot SPI interface.

In order for it to work it needs access to certain E1000-internal
functions, so export those in the e1000.h header file.

Signed-off-by: Kyle Moffett <Kyle.D.Moffett@boeing.com>
Cc: Ben Warren <biggerbadderben@gmail.com>