spi: rtio: Extract common APIs into separate file

Extracted common SPI RTIO operations from the spi_mcux_lpspi driver
into spi_rtio, which should be common across RTIO drivers.

Tested with spi_loopback with and without CONFIG_SPI_RTIO. Ran on
mimxrt1010_evk.

Also, verified the other SPI RTIO driver (spi_sam) is not broken by
these changes (tested building for target: robokit1 with the same
conditions as above).

Signed-off-by: Luis Ubieda <luisf@croxel.com>