Documentation: devres: add missing PWM helper

Documentation: devres: add missing PWM helper

Add missing devm_pwmchip_add() to devres.rst. It's introduced by
commit bcda91bf86c1 ("pwm: Add a device-managed function to add
PWM chips").

Fixes: bcda91bf86c1 ("pwm: Add a device-managed function to add PWM chips")
Cc: Thierry Reding <thierry.reding@gmail.com>
Cc: "Uwe Kleine-König" <u.kleine-koenig@pengutronix.de>
Cc: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Link: https://lore.kernel.org/r/20221102024430.1444714-1-yangyingliang@huawei.com
Signed-off-by: Jonathan Corbet <corbet@lwn.net>