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>