manifest:default: Add Mediatek layers

This adds all the layers necessary to support Mediatek SoCs. For now, we need
two poky versions because the upstream Mediatek layers are not compatible with
our NXP poky revision.

Original commit by Oleksii Kutuzov <oleksii.kutuzov@seco.com>

Change-Id: I42b2e81b937a1d45cc0f1bc9837e05bc83b36913