Add rust-based flash driver for the "STM32F746ZG" board

[UdayakumarHidakal]

In order to port rustBoot to the STM32F746ZG board, we'll need to implement a flash-driver written in rust.

Sub-tasks:

• The flash driver should support lock and unlock flash operations.
• The flash driver must write and erase data-blocks from the device's internal flash memory.
• Finally, it must implement rustBoot's FlashInterface trait.

Additional notes:

• You only need to implement the methods (of FlashInterface trait), applicable to your board.