Documentation: e100, e1000: Add missing SPDX header

Add the SPDX-Lincense-Identifier to the Intel wired Ethernet *.rst
kernel documentation.

Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>