arm: dts: mt7623: Update ethsys binding

commit 76a09ce214addb8ddc0f6d50dc1106a5f829e713 upstream.

The ethsys binding misses the reset-cells, this patch
adds this property.

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>