dt-bindings: Add isl68137 as a trivial device

The isl68137 is a digital output 7-phrase configurable PWM controller
with an AVSBus interface from Intersil.

Signed-off-by: Patrick Venture <venture@google.com>
Acked-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Rob Herring <robh@kernel.org>

Documentation/devicetree/bindings/trivial-devices.yaml

@@ -104,6 +104,8 @@ properties:
           - isil,isl29028
             # Intersil ISL29030 Ambient Light and Proximity Sensor
           - isil,isl29030
+            # Intersil ISL68137 Digital Output Configurable PWM Controller
+          - isil,isl68137
             # 5 Bit Programmable, Pulse-Width Modulator
           - maxim,ds1050
             # Low-Power, 4-/12-Channel, 2-Wire Serial, 12-Bit ADCs