ARM: Kconfig: Introduce MACH_STM32F746 flag

This patch introduces the MACH_STM32F746 to make possible to only select
STM32F746 pinctrl driver

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

Signed-off-by: Alexandre TORGUE <alexandre.torgue@st.com>