ARM: dts: am335x-lxm: Use rmii-clock-ext

Use external clock for RMII since the internal clock doesn't meet the
jitter requirements.

Signed-off-by: George McCollister <[e-mail]>
Signed-off-by: Tony Lindgren <[e-mail]>

arch/arm/boot/dts/am335x-lxm.dts

···
 	dual_emac_res_vlan = <3>;
 };
 
+&phy_sel {
+	rmii-clock-ext;
+};
+
 &mac {
 	pinctrl-names = "default", "sleep";
 	pinctrl-0 = <&cpsw_default>;