Binding for sbs-manager

Required properties:
- compatible: "<vendor>,<part-number>", "sbs,sbs-charger" as fallback. The part
  number compatible string might be used in order to take care of vendor
  specific registers.
- reg: integer, i2c address of the device. Should be <0xa>.
Optional properties:
- gpio-controller: Marks the port as GPIO controller.
  See "gpio-specifier" in .../devicetree/bindings/gpio/gpio.txt.
- #gpio-cells: Should be <2>. The first cell is the pin number, the second cell
  is used to specify optional parameters:
  See "gpio-specifier" in .../devicetree/bindings/gpio/gpio.txt.

From OS view the device is basically an i2c-mux used to communicate with up to
four smart battery devices at address 0xb. The driver actually implements this
behaviour. So standard i2c-mux nodes can be used to register up to four slave
batteries. Channels will be numerated starting from 1 to 4.

Example:

batman@a {
    compatible = "lltc,ltc1760", "sbs,sbs-manager";
    reg = <0x0a>;
    #address-cells = <1>;
    #size-cells = <0>;

    gpio-controller;
    #gpio-cells = <2>;

    i2c@1 {
        #address-cells = <1>;
        #size-cells = <0>;
        reg = <1>;

        battery@b {
            compatible = "ti,bq2060", "sbs,sbs-battery";
            reg = <0x0b>;
            sbs,battery-detect-gpios = <&batman 1 1>;
        };
    };

    i2c@2 {
        #address-cells = <1>;
        #size-cells = <0>;
        reg = <2>;

        battery@b {
            compatible = "ti,bq2060", "sbs,sbs-battery";
            reg = <0x0b>;
            sbs,battery-detect-gpios = <&batman 2 1>;
        };
    };

    i2c@3 {
        #address-cells = <1>;
        #size-cells = <0>;
        reg = <3>;

        battery@b {
            compatible = "ti,bq2060", "sbs,sbs-battery";
            reg = <0x0b>;
            sbs,battery-detect-gpios = <&batman 3 1>;
        };
    };
};