Specifying Voltage Domain information for devices
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1. Specifying a voltage domain.

SoCs may have multiple voltage domains under which various clocks operate.

Mandatory Properties:
- #voltdm-cells - indicates if there are specifiers needed to reference the
  voltage domain

Optional Properties:
- voltage-tolerance: Specify the voltage tolerance in percentage

2. Voltage domain consumer
consumer nodes can be reference using the below binding:
- <name>-voltdm: phandle to the voltage domain node.

Examples:

A. Voltage Domain controlling multiple regulator
voltage_domain_mpu: voltdm@1 {
	compatible = "xyz";
	#voltdm-cells = <0>;
	vdd-supply = <&vcc>;
	vbb-supply = <&abb_mpu>;
	...
};

voltage_domain_gpu: voltdm@2 {
	compatible = "xyz";
	#voltdm-cells = <0>;
	vdd-supply = <&dcdc2>;
	vbb-supply = <&abb_gpu>;
	...
};
...
&cpu0 {
	cpu0-voltdm = <&voltage_domain_mpu>;
	voltage-tolerance = <1>;
};

&gpu {
	gpu-voltdm = <&voltage_domain_gpu>;
};

B. Indexed voltage domain device

#define SOC_XYZ_VOLTAGE_DOMAIN_MPU	0
#define SOC_XYZ_VOLTAGE_DOMAIN_GFX	1

voltage_domain_socx: voltdm@1 {
	compatible = "abc";
	#voltdm-cells = <1>;
	...
};
...
&cpu0 {
	cpu0-voltdm = <&voltage_domain_socx SOC_XYZ_VOLTAGE_DOMAIN_MPU>;
	voltage-tolerance = <1>;
};

&gpu {
	gpu-voltdm = <&voltage_domain_socx SOC_XYZ_VOLTAGE_DOMAIN_GFX>;
};
