net: mvmdio: add clocks property to binding documentation

Commit 3d604da1
  ("net: mvmdio: get and enable optional clock")

was missing an update of the corresponding device tree binding
documentation. This patch adds the clocks property to mvmdio
binding documentation.

Signed-off-by: Sebastian Hesselbarth <sebastian.hesselbarth@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>