ARM: imx: imx8mn-*-evk: use DM settings for PHY configuration

ARM: imx: imx8mn-*-evk: use DM settings for PHY configuration

With the correct settings described in the device-tree the PHY settings
in the board init are no longer required. The values are taken from the
linux device tree.

The PHY latency settings are derived from the phy-mode property and the
voltage seetings are done via the regulator.

Suggested-by: Michael Walle <michael@walle.cc>
Signed-off-by: Heiko Thiery <heiko.thiery@gmail.com>
Tested-by: Marek Vasut <marex@denx.de> # 8MNANOD4-EVK
Reviewed-by: Fabio Estevam <festevam@gmail.com>