pwm: dwc: Use devm_pwmchip_add()

Add the PWM chip using devm_pwmchip_add() to avoid having to manually
remove it. This is useful for subsequent patches adding platform device
support.

Signed-off-by: Ben Dooks <ben.dooks@sifive.com>
Reviewed-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>