dt-bindings: net: dsa: qca8k: document reset-gpios property

This patch documents the qca8k's reset-gpios property that
can be used if the QCA8337N ends up in a bad state during
reset.

Signed-off-by: Christian Lamparter <chunkeey@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>