pinctrl: mvebu: remove the offset property for regmap

The offset property of the pinctrl node, when a regmap is used in the
device tree, was never used nor documented in the binding. Moreover, the
compatible string is enough to let the driver know which offset using.

So this patch removes the property and move the information at the driver
level.

Reviewed-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>