gpio: mvebu: switch pwm duration registers to regmap

Commit 2233bf7a ("gpio: mvebu: switch to regmap for register access")
changed most readl/writel registers access calls to the regmap API in
preparation for Armada 7K/8K support. PWM duration registers were left using
readl/writel, as the driver does not support PWM for Armada 7K/8K.

Switch PWM duration registers to regmap as first step in adding Armada 7K/8K
PWM functionality support.

Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>