spi: mcux_lpspi: Refactor driver to extract common RTIO functionality

As a step to make them common code: spi_rtio.c.
Verified this refactorization builds and passes spi_loopback, both with
CONFIG_SPI_RTIO enabled, as well as disabled. Tested on mimxrt1010_evk.

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