regulator/of_get_regulator: add child path to find the regulator supplier

when the VIR_LDO1 regulator supplier is it's brother,
we can't find the supplier.

example code :
&vir_regulator {
	ldo0_vir: ldo0-virtual {
		regulator-compatible = "VIR_LDO0";
		regulator-name= "VIR_LDO0";
		regulator-min-microvolt = <1000000>;
		regulator-max-microvolt = <2000000>;
	};
	ldo1_vir: ldo1-virtual {
		regulator-compatible = "VIR_LDO1";
		regulator-name= "VIR_LDO1";
		regulator-min-microvolt = <1000000>;
		regulator-max-microvolt = <3000000>;
		ldo1-supply = <&ldo0_vir>;
	};
	...
}

so we add the child ptah to find the suppier.

Signed-off-by: zoro <long17.cool@163.com>
Signed-off-by: Mark Brown <broonie@kernel.org>