ARM: dts: STiH407-pinctrl: Update gpio-cells to 2

This patch allows to use second parameter to the gpio
specifier, which is used to specify whether the gpio is
active high or low.

Signed-off-by: Patrice Chotard <patrice.chotard@st.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>