dt-bindings: ti-lmu: Modify dt bindings for the LM3697

The LM3697 is a single function LED driver. The single function LED
driver needs to reside in the LED directory as a dedicated LED driver
and not as a MFD device.  The device does have common brightness and ramp
features and those can be accomodated by a TI LMU framework.

The LM3697 dt binding needs to be moved from the ti-lmu.txt and a dedicated
LED dt binding needs to be added.  The new LM3697 LED dt binding will then
reside in the Documentation/devicetree/bindings/leds directory and follow the
current LED and general bindings guidelines.

Signed-off-by: Dan Murphy <dmurphy@ti.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Jacek Anaszewski <jacek.anaszewski@gmail.com>