soc: st: set soc-level phy priority

Introducing MDIO and PHY support for stm32, phy driver gets
error (-116) if it tries to read phy chip id, since MDIO IP is
part of ETH IP, and eth hw module is still not initialized.

Forcing a priority that allows possibly connected PHY chip to be
detected properly at initial boot.

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