dts: am335x-brsmarc1/xre1: insert phy_id again

dts: am335x-brsmarc1/xre1: insert phy_id again

commit dde06d52cb00
("arm: dts: am335x: sync cpsw/mdio/phy with latest linux - drop phy_id")

did sync with recent linux kernel and replaced therefore the 'phy_id'
property with a phy-handle pointing to the mdio.

This is OK for linux, but introduces trouble with the already running
vxWorks on this target.

So this commit here re-inerts the phy_id property beside the phy-handle
property to be compatible with both.

Signed-off-by: Hannes Schmelzer <hannes.schmelzer@br-automation.com>