Sandbox, PMIC regulators

This device uses two drivers:
- drivers/power/pmic/sandbox.c (as parent I/O device)
- drivers/power/regulator/sandbox.c (for child regulators)

This file describes the binding info for the REGULATOR driver.

First, please read the binding info for the PMIC:
- doc/device-tree-bindings/pmic/sandbox.txt

Required subnodes:
- ldoN { };
- buckN { };

The sandbox PMIC can support: ldo1, ldo2, buck1, buck2.

For each PMIC's regulator subnode, there is one required property:
- regulator-name: used for regulator uclass platform data '.name'

Optional:
- regulator-min-microvolt: minimum allowed Voltage to set
- regulator-max-microvolt: minimum allowed Voltage to set
- regulator-min-microamps: minimum allowed Current limit to set (LDO1/BUCK1)
- regulator-max-microamps: minimum allowed Current limit to set (LDO1/BUCK1)
- regulator-always-on: regulator should be never disabled
- regulator-boot-on: regulator should be enabled by the bootloader

Example PMIC's regulator subnodes:

ldo1 {
	regulator-name = "VDD_1.0V";
	regulator-min-microvolt = <1000000>;
	regulator-max-microvolt = <1200000>;
	regulator-min-microamps = <100000>;
	regulator-max-microamps = <400000>;
	regulator-always-on;
};

buck2 {
	regulator-name = "VDD_1.8V";
	regulator-min-microvolt = <1800000>;
	regulator-max-microvolt = <1800000>;
	regulator-boot-on;
};