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.

(cooloney@gmail.com: fix typo in the subject)

Signed-off-by: NeilBrown <neilb@suse.de>
Signed-off-by: Bryan Wu <cooloney@gmail.com>