staging: mt7621-pci-phy: change driver to don't use child nodes

Device tree has been simplified to don't use child nodes and use
the #phy-cells property instead. Change the driver accordly implementing
custom 'xlate' function to return the correct phy for each port.

Signed-off-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Reviewed-by: NeilBrown <neil@brown.name>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>