Add way to mark a peripheral as read-only

We've got two tasks on the SP sharing access to the "system flash" region to read out unique ID. That ROM also contains calibration information for several peripherals, so this will not be the last time we run into this.

While we generally discourage sharing of peripherals across tasks, there's no technical mechanism that prevents it, and in this case it's desirable. We could make this safer by adding a way to designate a "peripheral" (here, a fixed named ROM rather than registers) as read-only.

No files or tests are named. Start by tracing how Hubris designates peripherals and enforces access between tasks, then examine the existing sharing behavior for the system flash region. Done means a peripheral can be explicitly designated read-only while preserving safe access for multiple tasks, with coverage for the relevant access rules.