ARM: dts: am335x-sl50: use phy-phandle declarations

phy-phandle is now a preferred method to reference a PHY device. The new
method also allows you to specify a reset gpio which is required for
this board.

Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>