leds: ktd2692: move ExpressWire code to library

leds: ktd2692: move ExpressWire code to library

The ExpressWire protocol is shared between at least KTD2692 and KTD2801
with slight differences such as timings and the former not having a
defined set of pulses for enabling the protocol (possibly because it
does not support PWM unlike KTD2801). Despite these differences the
ExpressWire handling code can be shared between the two, so move it into
a library in preparation for adding KTD2801 support.

Suggested-by: Daniel Thompson <daniel.thompson@linaro.org>
Signed-off-by: Duje Mihanović <duje.mihanovic@skole.hr>