mediatek: Port AV changes

This ports the changes required to perform video decoding
and enconding.

The changes are ported from the mediatek BSP for mt6592
with the minimum required feature set and confined to
allow co-existance with changes from other vendors.

[Trimmed down for L]

Change-Id: I3709de0e5b9e4e0f68a71e182549e72a3dab26a7