ARM: Kconfig: Introduce MACH_STM32F429 flag

This patch introduces the MACH_STM32F429 to make possible to only select
STM32F429 pinctrl driver.

By default, all the MACH_STM32Fxxx flags will be set with STM32 defconfig.

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