dt-bindings: leds: Convert PCA9532 to dtschema

Convert the PCA9532 LED Dimmer to dtschema.
While at it, update the example to match recommended node names and
the link to the product datasheet. Also add GPIO properties since
the driver allows to use unused pins as GPIOs.

Signed-off-by: Wadim Egorov <w.egorov@phytec.de>
Acked-by: Pavel Machek <pavel@ucw.cz>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Lee Jones <lee@kernel.org>
Link: https://lore.kernel.org/r/20230412140552.451527-1-w.egorov@phytec.de