dt-bindings: leds: maxim,max77693: Convert to dtschema

Convert the LEDs bindings of Maxim MAX77693 MUIC to DT schema format.
The existing bindings were defined in ../bindings/mfd/max77693.txt.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>