ARM: stm32: Introduce MACH_STM32F469 flag

This patch introduces the MACH_STM32F469 to make possible to only select
STM32F469 pinctrl driver

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

Signed-off-by: Alexandre TORGUE <alexandre.torgue@st.com>
Signed-off-by: Olof Johansson <olof@lixom.net>