dt-bindings: reset: Add TPS380x documentation

Add device-tree binding documentation for the tps380x reset driver. The
binding uses enum to make it easy to add more devices from that family.

Signed-off-by: Marco Felsch <m.felsch@pengutronix.de>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Link: https://lore.kernel.org/r/20220530092226.748644-1-m.felsch@pengutronix.de