boards: stm32: stm32n6570_dk: fix DTS errors for Ethernet

The DTS for this board contained a few mistakes:
* The PHY address was incorrect
	* This did not impair functionality because the RTL8211F
	  treats address 0 as "broadcast" and accepts commands
	  sent to it by default
* An incorrect PHY driver was used
	* The generic "phy_mii" driver is not the most appropriate
	  for this PHY - use the HW-specific RTL8211F driver instead
	* Also remove the PHY IRQ pin configuration from Ethernet
	  pinctrl to allow RTL8211F driver to receive IRQ via GPIO
	  instead
* The RGMII GTX CLK pinctrl must be configured at "medium-speed"
	* This is a temporary solution. The proper solution would use
	  very-high-speed combined with a GPIO delay, but delays are
	  not supported yet in Zephyr.

Signed-off-by: Mathieu CHOPLAIN <mathieu.choplain@st.com>