gpio: mvebu: add pwm support for Armada 8K/7K

Use the marvell,pwm-offset DT property to store the location of PWM
signal duration registers.

Since we have more than two GPIO chips per system, we can't use the
alias id to differentiate between them. Use the offset value for that.

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>