Add documentation for 'xdpe122' devices.
Signed-off-by: Vadim Pasternak <vadimp@mellanox.com>
Link: https://lore.kernel.org/r/20200113150841.17670-7-vadimp@mellanox.com
[groeck: Added to index.rst]
Signed-off-by: Guenter Roeck <linux@roeck-us.net>