devicetree: add lm90 thermal_zone sensor support

This patch updates the LM90's devicetree definition to
include the #thermal-sensor-cells property as well as
the sensor constants in include/dt-bindings/thermal/lm90.h.

Cc: Wei Ni <wni@nvidia.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Christian Lamparter <chunkeey@googlemail.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>