regulator: map consumer regulator based on device tree

Device nodes in DT can associate themselves with one or more
regulators/supply by providing a list of phandles (to regulator nodes)
and corresponding supply names.

For Example:
	devicenode: node@0x0 {
		...
		...
		vmmc-supply = <&regulator1>;
		vpll-supply = <&regulator2>;
	};

The driver would then do a regulator_get(dev, "vmmc"); to get
regulator1 and do a regulator_get(dev, "vpll"); to get
regulator2.

of_get_regulator() extracts the regulator node for a given
device, based on the supply name.

Use it to look up the regulator for a given consumer from device tree, during
a regulator_get(). If not found fallback and lookup through
the regulator_map_list instead.

Also, since the regulator dt nodes can use the same binding to
associate with a parent regulator/supply, allow the drivers to
specify a supply_name, which can then be used to lookup dt
to find the parent phandle.

Signed-off-by: Rajendra Nayak <rnayak@ti.com>
Acked-by: Grant Likely <grant.likely@secretlab.ca>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>