dt-bindings: arm: rockchip: remove reference to fennec board

The rk3288 fennec board has been removed, remove the binding document at
the same time.

Signed-off-by: Kever Yang <kever.yang@rock-chips.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>