drivers: spi: mcux_flexcomm: make async mode with dma work

The current driver implementation would block even when the async
API was invoked, so it wasn't really async.

This CL also fully chains the DMA transfer using multiple dma blocks
and makes the number of dma blocks available a config value. The
increase in number of dma blocks is needed so that a spi_buf_set
that has many entries can be converted into chained dma transfers
with the last transfer in a separate block that will set the EOT flag.

Also make some improvements:
1) When doing single cnt transfer, don't use the SDK driver but
   use a new fucntion spi_mcux_transfer_single_word().
   It's much more efficient and does not use an ISR
   like the SDK function does just to send one word.

2) Fix calls to spi_context_update_tx/rx() so that the
   correct word size is passed in, instead of previously
   being hardcoded to 1. This only matters when word size
   is two. Evaluate the word_size_bytes and word_size_bits
   once and store the values in data instead of computing
   it multiple times in various parts of the driver

3) When CONFIG_SPI_MCUX_FLEXCOMM_DMA is defined, we
   do not use the IRQ handler, so add #ifdefs to compile
   that code out. This reduces code size.

Signed-off-by: Mike J. Chen <mjchen@google.com>