Documentation: dt: Remove ngpios from stm32-pinctrl binding

Remove "ngpios" bindings definition as it is no more used in stm32 pinctrl
driver.

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>