dt-bindings: gpio: gpio-mvebu: document offset and marvell,pwm-offset

The offset and marvell,pwm-offset properties weren't in the old binding.
Add them based on the existing usage in the driver and board DTS when
the marvell,armada-8k-gpio compatible is used.

Signed-off-by: Chris Packham <chris.packham@alliedtelesis.co.nz>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Acked-by: Thierry Reding <thierry.reding@gmail.com>
Signed-off-by: Bartosz Golaszewski <brgl@bgdev.pl>