[WIP] sof: ipc: switch platform IPC to Zephyr service

[tmleman]

This patch reworks the SOF IPC platform integration to use the generic Zephyr ipc_service instead of the Intel audio DSP specific driver.

Before this change SOF was talking directly to the Intel ADSP IPC driver, which made the IPC path tightly coupled to that particular backend. All commands were sent and completed via intel_adsp_ipc_*() functions.

The code now sends and receives IPC commands through a Zephyr ipc_service endpoint registered on the Intel ADSP host IPC instance, using sof_ipc_receive_cb() as the receive handler. Incoming messages are processed as before using the existing compact IPC path to process commands.

Each IPC command is treated as a compact two-word ipc_cmd_hdr and a BUILD_ASSERT guarantees that the header size remains aligned with the transport format assumptions.

This change is part of ongoing work to better integrate SOF with Zephyr and will allow other vendors to more easily integrate their own IPC backends.