Enable pure YAML based workflow

[maximilian-maisel-bl]

Currently, chiptool generated PAC are missing top level peripherals and interrupt
definitions when used from a pure YAML workflow, i.e. without an SVD file, as in
the following command:

chiptool gen-block --input xmc4800.yml --output src/lib.rs

Such an approach is useful if no well-written SVD file is available for
the target MCU. For example, the XMC4800 SVD file calls all enumerated values
value1, value2 and so on resulting in a de-facto unusable PAC.

Since all IR already implements Serialize and Deserialize traits,
this workflow is enabled by reading/writing the IR Device block
from/to YAML as well as done in the PR.

A minimum working YAML looks like:

---
device/:
  nvic_priority_bits: 4
  peripherals:
  - name: SCU
    description: System Control Unit
    base_address: 0x50004000
    block: scu::Scu
  # [...]
  interrupts:
  - name: SCU_0
    description: System Control
    value: 0
  # [...]

# Below is chiptool YAML as usual
block/scu::Scu:
  description: System Control Unit
  items:
  - name: id
    description: ID register
    byte_offset: 0
    fieldset: scu::regs::Id
  - name: mirrsts
    description: Mirror Write Status Register
    byte_offset: 0xC4

fieldset/scu::regs::Id:
  description: ID register
  fields:
  - name: MOD_NUMBER
    description: Module Number
    bit_offset: 16
    bit_size: 16
#[...]

Without this PR, this YAML fails to load with
Unknown kind "device" at line 2 column 1.

Is this a suitable approach or did I miss a reason
why Device was intentionally not read/written from/to YAML?