dt/bindings: fix documentation of ethernet-phy compatible property

A recent commit extended the documentation of the ethernet-phy
compatible property, but placed the new paragraph under the max-speed
property.

Fixes: f00e756e ("dt: Document a compatible entry for MDIO ethernet
Phys")
Cc: devicetree@vger.kernel.org
Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>