Documentation: arm: stm32: Add stm32h750 value line doc

This patchset add support for soc stm32h750, stm32h750 has mirror
different from stm32h743

item            stm32h743     stm32h750
flash size:     2MiB          128KiB
adc:            none          3
crypto-hash:    none          aes/hamc/des/tdes/md5/sha

detail information can be found at:
https://www.st.com/en/microcontrollers-microprocessors/stm32h750-value-line.htmlSigned-off-by: dillon min <dillon.minfei@gmail.com>
Signed-off-by: Alexandre Torgue <alexandre.torgue@foss.st.com>

Documentation/arm/index.rst

@@ -52,6 +52,7 @@ SoC-specific documents
    stm32/stm32f746-overview
    stm32/overview
    stm32/stm32h743-overview
+   stm32/stm32h750-overview
    stm32/stm32f769-overview
    stm32/stm32f429-overview
    stm32/stm32mp157-overview

Documentation/arm/stm32/stm32h750-overview.rst

+==================
+STM32H750 Overview
+==================
+
+Introduction
+------------
+
+The STM32H750 is a Cortex-M7 MCU aimed at various applications.
+It features:
+
+- Cortex-M7 core running up to @480MHz
+- 128K internal flash, 1MBytes internal RAM
+- FMC controller to connect SDRAM, NOR and NAND memories
+- Dual mode QSPI
+- SD/MMC/SDIO support
+- Ethernet controller
+- USB OTFG FS & HS controllers
+- I2C, SPI, CAN busses support
+- Several 16 & 32 bits general purpose timers
+- Serial Audio interface
+- LCD controller
+- HDMI-CEC
+- SPDIFRX
+- DFSDM
+
+Resources
+---------
+
+Datasheet and reference manual are publicly available on ST website (STM32H750_).
+
+.. _STM32H750: https://www.st.com/en/microcontrollers-microprocessors/stm32h750-value-line.html
+
+:Authors: Dillon Min <dillon.minfei@gmail.com>
+