dt-bindings: ARM: Mediatek: Fix ethsys documentation

The ethsys registers a reset controller, so we need to specify a
reset cell. This patch fixes the documentation.

Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Stephen Boyd <sboyd@codeaurora.org>