spi: SAM add RTIO support

Implements the SPI RTIO API. Refactors many of the internal transfer
functions to work on simple types to allow for both the RTIO and existing
SPI API functions to co-exist.

Removes the unneeded shift_master specialization. Either a polling or DMA
transfer flow is used when used with RTIO as all operations are normalized.

When SPI_RTIO is enabled the spi_transceive blocking call translates
the request into an RTIO transaction placed in the queue of transactions
the device is meant to do.

Signed-off-by: Tom Burdick <thomas.burdick@intel.com>