ARM: BCM5301X: Specify PHY of USB 2.0 in DT

Driver for Northstar USB 2.0 PHY was added in 4.7-rc1 by:
commit d3feb406 ("phy: bcm-ns-usb2: new driver for USB 2.0 PHY on
Northstar").
It should be used to let EHCI platform driver init PHY.

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>