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Rockchip EMMC PHY
-----------------------

Required properties:
 - compatible: rockchip,rk3399-emmc-phy
 - rockchip,grf : phandle to the syscon managing the "general
   register files"
 - #phy-cells: must be 0
 - reg-offset: PHY configure reg address offset in "general
   register files"
 - ctrl-base: controller digital block's physical address.

Optional Properties:
 - freq-sel: must match the freq of emmc clock, only support the
   following frequency: 50000000, 100000000, 150000000, 200000000.
   If not assigned any freq, default use 200000000Hz.
 - dr-sel: select the drive strength of phy IO.
 - opdelay: select the output delay count number for clk IO and data IO
 - 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:

emmcphy: phy {
	compatible = "rockchip,rk3399-emmc-phy";
	rockchip,grf = <&grf>;
	reg-offset = <0xf780>;
	ctrl-base = <0xfe330000>;
	#phy-cells = <0>;
};