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>

Documentation/networking/e100.rst

-==============================================================
+.. SPDX-License-Identifier: GPL-2.0+
+
 Linux* Base Driver for the Intel(R) PRO/100 Family of Adapters
 ==============================================================
 

Documentation/networking/e1000.rst

-===========================================================
+.. SPDX-License-Identifier: GPL-2.0+
+
 Linux* Base Driver for Intel(R) Ethernet Network Connection
 ===========================================================