dt-bindings: pwm: imx: add i.MX8QXP compatible

i.MX8QXP compatible is missing in the list, add it.

Signed-off-by: Alexander Stein <alexander.stein@ew.tq-group.com>
Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>