ARM: dts: am57xx-beagle-x15: implement errata "Ethernet RGMII2 Limited to 10/100 Mbps"

According to errata i880 description the speed of Ethernet port 1 on AM572x
SoCs rev 1.1 should be limited to 10/100Mbps, because RGMII2 Switching
Characteristics are not compatible with 1000 Mbps operation [1].
The issue is fixed with Rev 2.0 silicon.

Hence, rework Beagle-X15 and Begale-X15-revb1 to use phy-handle instead of
phy_id and apply corresponding limitation to the Ethernet Phy 1.

[1] http://www.ti.com/lit/er/sprz429j/sprz429j.pdf



Signed-off-by: Grygorii Strashko <grygorii.strashko@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>