LEDS: tca6507: add device-tree support for GPIO configuration.
The 7 lines driven by the TCA6507 can either drive LEDs or act as output-only
GPIOs.
To make this distinction in devicetree we use the "compatible" property.
If the device attached to a line is "compatible" with "gpio", we treat it
like a GPIO. If it is "compatible" with "led" (or if no "compatible" value
is set) we treat it like an LED.