regulator: Fix up a confusing dev_warn when DT lookup fails

of_parse_phandle() returns NULL either if the property name
itself does not exist or if it (exists and) does not
reference a valid phandle.
Giving out a warn like the one below (that the property references
an invalid phandle) can be confusing when the property itself
does not exist in the node.
Fix it with a more sensible message and make it a dev_dbg instead
of a dev_warn.

Reported-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Signed-off-by: Rajendra Nayak <rnayak@ti.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>

drivers/regulator/core.c

@@ -148,7 +148,7 @@ static struct device_node *of_get_regulator(struct device *dev, const char *supp
 	regnode = of_parse_phandle(dev->of_node, prop_name, 0);
 
 	if (!regnode) {
-		dev_warn(dev, "%s property in node %s references invalid phandle",
+		dev_dbg(dev, "Looking up %s property in node %s failed",
 				prop_name, dev->of_node->full_name);
 		return NULL;
 	}