Remove MPU, cache initialization

[mciantyre]

The PR removes scope from the BSP. The BSP will no longer define MPU regions, nor enable I- or D-Cache. Users may re-enable these features on their own.

[mciantyre]

bors try

[bors[bot]]

try

Build failed:

• clippy

[mciantyre]

bors r+

[bors[bot]]

Build succeeded:

• clippy
• doc
• doclinks
• features
• features (rtic)
• features (rtic,rt)
• features (systick)
• features (systick,rt)
• features (usb-logging)
• features (usb-logging,rt)
• format
• precompiled
• template
• test
• xplat (macos-latest)
• xplat (ubuntu-latest)
• xplat (windows-latest)