drivers: led_strip: add public API for addressable LED strips

This API covers drivers for strips, or strings, of individually
addressable LEDs. Both RGB and grayscale LED strip drivers can be
implemented within these APIs.

The API only provides for updating the entire strip, since not all
strips support updating individual LEDs without affecting the others.

Subsequent patches will add individual driver support.

Signed-off-by: Marti Bolivar <marti.bolivar@linaro.org>