pinctrl: st: Use second parameter to gpio specifier

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

Simply remove specific of_xlate callback() and of_gpio_n_cells
and use default one set by of_gpiochip_add() which allows
to use second parameter gpio specifier.

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