dtlib: use IntEnum for marker types

Add a _MarkerType enum. A subsequent commit will use it for type
annotations in the Property class.

Fix an incorrect type in a comment while we're here.

We continue to use 'marker_type is _MarkerType.FOO' instead of
'marker_type == _MarkerType.FOO' because we are adding those actual
_MarkerType.FOO objects to each property, so 'is' comparison
is legitimate.

Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>