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>