spi: spi-fsl-dspi: Use whole page for DMA buffers

dma_alloc_noncoherent() allocations are backed by a full page anyway, so
use it all.

VF610 devices used to use the full page before commit a957499b
("spi: spi-fsl-dspi: Fix bits-per-word acceleration in DMA mode"), but
others still used the FIFO size. After that commit, all devices used the
FIFO size. Now all devices use the full page.

Signed-off-by: James Clark <james.clark@linaro.org>
Reviewed-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Message-ID: <20250902-james-nxp-spi-dma-v6-5-f7aa2c5e56e2@linaro.org>
Signed-off-by: Mark Brown <broonie@kernel.org>