KSZ8051: register 0x1f, bits 5..4
See the respective PHY datasheet for the mode values.
+
+ - clocks, clock-names: contains clocks according to the common clock bindings.
+
+ supported clocks:
+ - KSZ8021, KSZ8031: "rmii-ref": The RMII refence input clock. Used
+ to determine the XI input clock.