pinctrl: Amend bindings for STM32 pinctrl

Adds "ngpios" and "gpio-ranges" bindings definition.

Signed-off-by: Alexandre TORGUE <alexandre.torgue@st.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>