Fixups to run on Nexus platform

- Update the linker scripts to reflect real memory addresses and sizes.
- Updates to some constants (cpu freq, peripheral clock, baud rates).
- Regenerate PLIC constants via regtool, to sync with the current HW.
- Add in a GPIO stub and debug writer interface, and replace the "panic
  panic panic" panic handler with Tock's real one: this gives really
  nice output if you do happen to hit a panic.

Bug: 258850990
Change-Id: I266b54e84b89bd8998f655702b01dfd85735321a