gpio: mvebu: Make use of devm_pwmchip_add

This allows to get rid of a call to pwmchip_remove() in the error path. There
is no .remove function for this driver, so this change fixes a resource leak
when a gpio-mvebu device is unbound.

Fixes: 757642f9 ("gpio: mvebu: Add limited PWM support")
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Reviewed-by: Andy Shevchenko <andy@kernel.org>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>