Add support for a QSPI enabled flash-driver for the RP2040 board

[nihalpasham]

In order to port rustBoot to the RP2040 board, we'll need to implement a flash-driver written in rust.

Sub-tasks:

• [ ] The flash driver must write and erase data-blocks from the device's external flash memory.
• [ ] More specifically, the RP2040 includes an Q-SPI flash chip, the flash driver must be a QSPI driver capable of performing block write and erase operations.
• [ ] Finally, it must implement rustBoot's FlashInterface trait.

Additional notes:

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

[abhishekdhamale]

Acknowledged

[sarath3192]

Acknowledged

[abhishekdhamale]

All sub-tasks are completed and PR #45 submitted.