pinctrl: remove maxpin from documentation

struct pinctrl_desc does not contain the maxpin member since commit 0d2006bb
(pinctrl: remove unnecessary max pin number).

Fixes: 0d2006bb ('pinctrl: remove unnecessary max pin number')
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>