dts: arm: st: add mdio node for h5 and h7

Add mdio node for h5 and h7 series.

Since MDIO registers are part of the same ETH hw IP, keeping mdio
node just as a child of mac/eth, cannot see as appropriate to assign
an adddress to it.

Signed-off-by: Angelo Dureghello <adureghello@baylibre.com>