gpio: mvebu: drop pwm base assignment

pwmchip_add() unconditionally assigns the base ID dynamically. Commit
f9a8ee8c ("pwm: Always allocate PWM chip base ID dynamically")
dropped all base assignment from drivers under drivers/pwm/. It missed
this driver. Fix that.

Fixes: f9a8ee8c ("pwm: Always allocate PWM chip base ID dynamically")
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Reviewed-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Bartosz Golaszewski <brgl@bgdev.pl>