dt-bindings: phy: Add a new property drive-impedance-ohm for RK's emmc PHY

 - reg: PHY register address offset and length in "general

   register files"

Optional clocks using the clock bindings (see ../clock/clock-bindings.txt),

specified by name:

Optional properties:

 - clock-names: Should contain "emmcclk".  Although this is listed as optional

		(because most boards can get basic functionality without having

		access to it), it is strongly suggested.

		See ../clock/clock-bindings.txt for details.

 - clocks: Should have a phandle to the card clock exported by the SDHCI driver.

 - drive-impedance-ohm: Specifies the drive impedance in Ohm.

                        Possible values are 33, 40, 50, 66 and 100.

                        If not set, the default value of 50 will be applied.

Example:

		reg = <0xf780 0x20>;

		clocks = <&sdhci>;

		clock-names = "emmcclk";

		drive-impedance-ohm = <50>;

		#phy-cells = <0>;

	};

};