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 <[e-mail]>
Reviewed-by: Krzysztof Kozlowski <[e-mail]>
Acked-by: Uwe Kleine-König <[e-mail]>
Acked-by: Thierry Reding <[e-mail]>
Signed-off-by: Bartosz Golaszewski <[e-mail]>

Documentation/devicetree/bindings/gpio/gpio-mvebu.yaml

···
       - const: pwm
     minItems: 1
 
+  offset:
+    $ref: /schemas/types.yaml#/definitions/uint32
+    description: Offset in the register map for the gpio registers (in bytes)
+
   interrupts:
     description: |
       The list of interrupts that are used for all the pins managed by this
···
 
   "#gpio-cells":
     const: 2
+
+  marvell,pwm-offset:
+    $ref: /schemas/types.yaml#/definitions/uint32
+    description: Offset in the register map for the pwm registers (in bytes)
 
   "#pwm-cells":
     description: