dt-bindings: net: ti: cpsw-switch: update to fix comments

After original patch was merged there were additional comments/requests
provided by Rob Herring [1]. Mostly they are related to json-schema usage,
and this patch fixes them. Also SPDX-License-Identifier has been changed to
(GPL-2.0-only OR BSD-2-Clause) as requested.

[1] https://lkml.org/lkml/2019/11/21/875


Fixes: ef63fe72 ("dt-bindings: net: ti: add new cpsw switch driver bindings")
Signed-off-by: Grygorii Strashko <grygorii.strashko@ti.com>
[robh: Remove 2 more maxItems that aren't necessary]
Signed-off-by: Rob Herring <robh@kernel.org>