arm64: introduce STM32 family on Armv8 architecture

Add a dedicated ARCH_STM32 for STM32 SoCs config. First STM32 Armv8 SoC
family is the STM32MP25 which is composed of STM32MP251, STM32MP253,
STM32MP255, STM32MP257 SoCs.
Signed-off-by: Alexandre Torgue <alexandre.torgue@foss.st.com>

arch/arm64/Kconfig.platforms

@@ -277,6 +277,20 @@ config ARCH_INTEL_SOCFPGA
 	  Stratix 10 (ex. Altera), Stratix10 Software Virtual Platform,
 	  Agilex and eASIC N5X.
 
+config ARCH_STM32
+	bool "STMicroelectronics STM32 SoC Family"
+	select GPIOLIB
+	select PINCTRL
+	select PINCTRL_STM32MP257
+	select ARM_SMC_MBOX
+	select ARM_SCMI_PROTOCOL
+	select COMMON_CLK_SCMI
+	help
+	  This enables support for ARMv8 based STMicroelectronics
+	  STM32 family, including:
+		- STM32MP25:
+			- STM32MP251, STM32MP253, STM32MP255 and STM32MP257.
+
 config ARCH_SYNQUACER
 	bool "Socionext SynQuacer SoC Family"
 	select IRQ_FASTEOI_HIERARCHY_HANDLERS