boards: arm: nucleo_f756zg: Created new board and soc definition

Added board definitions for nucleo_f756zg. Features include gpio,
pinmux, uart (ST Zio, ST-Link and Arduino Uno v3 interfaces).

Added basic documentation and some soc definitions for the
stm32 f756XX soc.

Signed-off-by: AJ Palmer <ajpcode@hotmail.com>

soc/arm/st_stm32/stm32f7/Kconfig.defconfig.stm32f756xx

@@ -0,0 +1,34 @@
+# Kconfig - ST STM32F756XX MCU configuration options
+#
+# Copyright (c) 2018 AJ Palmer
+#
+# SPDX-License-Identifier: Apache-2.0
+#
+
+if SOC_STM32F756XX
+
+config SOC
+	string
+	default "stm32f756xx"
+
+if GPIO_STM32
+
+config GPIO_STM32_PORTF
+	default y
+
+config GPIO_STM32_PORTG
+	default y
+
+config GPIO_STM32_PORTJ
+	default y
+
+config GPIO_STM32_PORTK
+	default y
+
+endif # GPIO_STM32
+
+config NUM_IRQS
+	int
+	default 98
+
+endif # SOC_STM32F756XX

soc/arm/st_stm32/stm32f7/Kconfig.soc

@@ -15,6 +15,9 @@ config SOC_STM32F723XE
 config SOC_STM32F746XG
 	bool "STM32F746XG"
 
+config SOC_STM32F756XX
+	bool "STM32F756XX"
+
 config SOC_STM32F769XI
 	bool "STM32F769XI"