Finally found a solution:
when i define the x:DataType="viewmodel:AddOcorrenciaViewModel"
in the contentpage all the child elements are set to my viewmodel, the problem is, the properties of my model arent in the viewmodel, they are inside the Model.
So what i did was:
I called my models in my content page: xmlns:models="clr-namespace:AppOcorrencias.MVVM.Models"
and then, inside the datatemplate i set the datatype for the model:
<CollectionView ItemsSource="{Binding OcorrenciaCustomFields}" Margin="0,10,0,0">
<CollectionView.ItemTemplate>
<DataTemplate x:DataType="models:CustomField">
<StackLayout Margin="0,5,0,0">
<Label Text="{Binding Name}" FontSize="12"/>
<Entry Text="{Binding Value, Mode=TwoWay}"
IsReadOnly="{Binding Type, Converter={StaticResource FieldTypeToViewConverter}, Mode=OneWay}"/>
</StackLayout>
</DataTemplate>
</CollectionView.ItemTemplate>
</CollectionView>
and this solved my problem.