boards: arduino: portenta_h7: Enable Ethernet and USB-HS Support.

Enable the mac node along with the necessary pinmuxing to enable the
ethernet and usb hs peripheral. I additionally defined the
`ethernet_phy_en` node the uses a fixed regulator to release
the PHY reset.

Additionally an usb_cdc_acm node was defined to enable serial over usb for
the console.

Signed-off-by: Rahul Arasikere <arasikere.rahul@gmail.com>