ARM: dts: am335x-baltos: use phy-phandle declarations

phy-phandle is now a preferred method to reference a PHY device.
Especially in regards to cpsw it enables PHY specific settings like
max-speed etc. being specified in DTS.

Signed-off-by: Yegor Yefremov <yegorslists@googlemail.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>