e1000: Allow direct access to the E1000 SPI EEPROM device

e1000: Allow direct access to the E1000 SPI EEPROM device

As a part of the manufacturing process for some of our custom hardware,
we are programming the EEPROMs attached to our Intel 82571EB controllers
from software using U-Boot and Linux.

This code provides several conditionally-compiled features to assist in
our manufacturing process:

  CONFIG_CMD_E1000:
    This is a basic "e1000" command which allows querying the controller
    and (if other config options are set) performing EEPROM programming.
    In particular, with CONFIG_E1000_SPI this allows you to display a
    hex-dump of the EEPROM, copy to/from main memory, and verify/update
    the software checksum.

  CONFIG_E1000_SPI_GENERIC:
    Build a generic SPI driver providing the standard U-Boot SPI driver
    interface.  This allows commands such as "sspi" to access the bus
    attached to the E1000 controller.  Additionally, some E1000 chipsets
    can support user data in a reserved space in the E1000 EEPROM which
    could be used for U-Boot environment storage.

  CONFIG_E1000_SPI:
    The core SPI access code used by the above interfaces.

For example, the following commands allow you to program the EEPROM from
a USB device (assumes CONFIG_E1000_SPI and CONFIG_CMD_E1000 are enabled):
  usb start
  fatload usb 0 $loadaddr 82571EB_No_Mgmt_Discrete-LOM.bin
  e1000 0 spi program $loadaddr 0 1024
  e1000 0 spi checksum update

Please keep in mind that the Intel-provided .eep files are organized as
16-bit words.  When converting them to binary form for programming you
must byteswap each 16-bit word so that it is in little-endian form.

This means that when reading and writing words to the SPI EEPROM, the
bit ordering for each word looks like this on the wire:

  Time >>>
------------------------------------------------------------------
  ... [7, 6, 5, 4, 3, 2, 1, 0, 15, 14, 13, 12, 11, 10, 9, 8], ...
------------------------------------------------------------------
  (MSB is 15, LSB is 0).

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