introduce enum types

[rohieb]

As per #3:

@jluebbe commented:

I'd prefer to have the enums be declared as types, but for now this is an improvement. We can change it later as we don't guarantee compatibility anyway. ;)

A enum type could look like this and allow reuse in the whole cluster:

clusters:
  PINCTRL:
    types:
      e2 CALIB:
        values:
          0: Disabled
          1: 1-step adjustment
          2: 2-step adjustment
          3: 3-step adjustment
      r32 EMI_ODT_CTRL:
        fields:
          e2 CALIB 26…27:
            name: ADDRESS_CALIB
          e2 CALIB 22…23:
            name: CONTROL_CALIB
          e2 CALIB 18…19:
            name: DUALPAD_CALIB