pinctrl: Add STM32 MCUs support

This patch adds pinctrl and GPIO support to STMicroelectronic's STM32
family of MCUs.

While it only supports STM32F429 for now, it has been designed to enable
support of other MCUs of the family (e.g. STM32F746).

Acked-by: Patrice Chotard <patrice.chotard@st.com>
Signed-off-by: Maxime Coquelin <mcoquelin.stm32@gmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>