soc/mediatek: Add DTS binding and definitions for AFE

Add a DTS binding for the MediaTek Audio Front End device, and
definitions for the in-tree devices.

These .dts files were auto-generated from pre-existing SOF code (that
defined the devices as C structs) using a tool currently being
submitted in the SOF tree, thus are included here as separate files.
The expectation is that future variants will be authored in this
format directly.  Longer term we can move them directly into the core
board DTS.

Signed-off-by: Andy Ross <andyross@google.com>