spi_nxp_lpspi: Fix DMA Async API

Fix the ASYNC DMA API on the lpspi driver and actually make the entire
driver go through that path. Rather than having an orthogonal
internally synchronous path we can just have both APIs go through the
same asynchronous path and just use wait_for_completion from spi context
to implement either sync or async.

Also make DMA driver default y if dependency (an lpspi having dmas
property) is met.

And lpspi_wait_tx_fifo_empty can be shared between drivers.

Signed-off-by: Declan Snyder <declan.snyder@nxp.com>