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: NAlexandre TORGUE <alexandre.torgue@st.com>

arch/arm/Kconfig

@@ -887,6 +887,11 @@ config MACH_STM32F429
 	depends on ARCH_STM32
 	default y
 
+config MACH_STM32F746
+	bool "STMicrolectronics STM32F746"
+	depends on ARCH_STM32
+	default y
+
 config ARCH_MPS2
 	bool "ARM MPS2 platform"
 	depends on ARM_SINGLE_ARMV7M