of: Improve of_phandle_iterator_next() error message

Understanding why of_phandle_iterator_next() returns an error can
sometimes be hard to decipher based solely on the error message, a
typical error example is that #foo-cells = <X> and the phandle property
used has a smaller number of cells specified, e.g.:

	#thermal-sensor-cells = <1>;
	phandle = <&scmi_sensor>

instead of:

	phandle <&scmi_sensor 0>;

Instead, make it clear what the expectations are towards debugging
incorrect Device Tree faster:

OF: /thermal-zones/scmi-thermal: #thermal-sensor-cells = 1, found 0 instead

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Rob Herring <robh@kernel.org>