[p2] Flash supports page write and other fixes

[YutingYou]

Problem

1. To improve firmware download speed, we need to make use of FLASH_TxData256B()

2. We're using 80MHz in the P2 flash clock configuration while MX25R32 requires up to 60MHz clock speed.

3. Minor fixes for SPI unit test

NOTE: This PR requires updating the prebootloader!

Steps to Test

1. Run usb-dfu to update DVOS to confirm the speed is improved
2. Update the prebootloader to observe the flash clock with an oscilloscope
3. Run the unit test wiring/no_fixture_spi

References

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Completeness

• User is totes amazing for contributing!
• Contributor has signed CLA (Info here)
• Problem and Solution clearly stated
• Run unit/integration/application tests on device
• Added documentation
• Added to CHANGELOG.md after merging (add links to docs and issues)

[avtolstoy]

@YutingYou Can you double check that the updates are going to work fine?

1. Update prebootloader with the change in this PR, use bootloader/system-part without this changeset
2. Update bootloader
3. Update system-part

[YutingYou]

@avtolstoy Double checked it with your steps to use the new prebootloader to update the old bootloader/system-part1 and new bootloader/system-part1. All worked fine. When updating the system-part1, both usb-dfu and particle flash --serial system-part1.bin worked for me.

[avtolstoy]

@YutingYou 🎉