docs: networking: device drivers: convert freescale/gianfar.txt to ReST

- add SPDX header;
- adjust titles and chapters, adding proper markups;
- use :field: markup;
- add to networking/index.rst.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>