spi: rspi: Remove unused 16-bit DMA support

spi: rspi: Remove unused 16-bit DMA support

The 16-bit DMA support doesn't fit well within the SPI core DMA framework,
as it needs to manage its own double-sized temporary buffers, for handling
the interleaved data.
Remove it, as there is no in-tree board code that sets
rspi_plat_data.dma_width_16bit.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Mark Brown <broonie@linaro.org>
(cherry picked from commit 9c5de2c1754c2bb3c69c4d7bf0d0edc0a61d8232)
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>