dt-bindings: net: Use phy-mode instead of phy-connection-type

When adding support for the DWMAC (and derivatives) bindings,
phy-connection-type was required, even though the previous binding required
the equivalent phy-mode.

Let's fix this by using phy-mode as we should have.

Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Signed-off-by: Rob Herring <robh@kernel.org>