spi: axi-spi-engine: add support for any word size

The AXI SPI Engine IP supports any word size from 1 to 32 bits. This
adds support for this by setting the bits_per_word_mask and emitting
the appropriate instruction to the SPI Engine each time a transfer
requires a new word size.

The functions that transfer tx/rx buffers from/to the SPI Engine
registers (spi_engine_write_{tx,rx}_fifo()) as well as the function that
creates the transfer instruction (spi_engine_gen_xfer()) also have to be
modified to take into account the word size since xfer->len is the
size of the buffers in bytes rather than words.

Signed-off-by: David Lechner <dlechner@baylibre.com>
Link: https://lore.kernel.org/r/20231117-axi-spi-engine-series-1-v1-14-cc59db999b87@baylibre.com
Signed-off-by: Mark Brown <broonie@kernel.org>

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