pinctrl: stm32: consider the GPIO offset to expose all the GPIO lines

pinctrl: stm32: consider the GPIO offset to expose all the GPIO lines

Consider the GPIO controller offset (from "gpio-ranges") to compute the
maximum GPIO line number.
This fixes an issue where gpio-ranges uses a non-null offset.
  e.g.: gpio-ranges = <&pinctrl 6 86 10>
        In that case the last valid GPIO line is not 9 but 15 (6 + 10 - 1)

Cc: stable@vger.kernel.org
Fixes: 67e2996f72c7 ("pinctrl: stm32: fix the reported number of GPIO lines per bank")
Reported-by: Christoph Fritz <chf.fritz@googlemail.com>
Signed-off-by: Fabien Dessenne <fabien.dessenne@foss.st.com>
Link: https://lore.kernel.org/r/20211215095808.621716-1-fabien.dessenne@foss.st.com
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>