Add `AddPeripherals` transform

[decaday]

Adds the AddPeripherals transform, allowing peripherals to be appended to matching devices.

Example:

  - !AddPeripherals
      devices: .*
      peripherals:
        - name: MAILBOX1
          base_address: 0x50082000
          block: mailbox::Mailbox

This follows the same motivation as the AddInterrupts transform.

The generic Add transform merges data at the top-level IR level. If Add were used to add a peripheral to an existing device, the IR::merge logic (or BTreeMap::extend) could cause the entire device entry to be replaced, overwriting the original lists of peripherals and interrupts for that device.