devlink: Add Documentation/networking/devlink-params-bnxt.txt

This patch adds a new file to add information about configuration
parameters that are supported by bnxt_en driver via devlink.

Cc: "David S. Miller" <davem@davemloft.net>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: linux-doc@vger.kernel.org
Cc: Jiri Pirko <jiri@mellanox.com>
Cc: Michael Chan <michael.chan@broadcom.com>
Signed-off-by: Vasundhara Volam <vasundhara-v.volam@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>