pinctrl: fix DT bindings for marvell,kirkwood-pinctrl

On Marvell mv88f6180 mpp pins range from 0 to 19 as well as from 35 to 44.
This is already fixed in commit: 9573e792

This is the documentation change for above commit.

Signed-off-by: Andreas Klinger <ak@it-klinger.de>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>