regulator: tps65910: dt: support when "regulators" node found

The device tree binding for the tps65910 is described as:
	tps65911 {
		reg = <0x2d>
		:::::::::
		regulators {
			compatible = "ti,tps65911";
			ldo1_reg: ldo1 {
				/** regulator entry */
			};

			ldo2_reg: ldo2 {
				/** regulator entry */
			};
			::::::::::
		};
	};

Support the regulators functionality only when there is "regulators"
child node available for tps65910.

Signed-off-by: Laxman Dewangan <ldewangan@nvidia.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>