spi: mt65xx: add dual and quad mode for standard spi device

spi: mt65xx: add dual and quad mode for standard spi device

Mediatek SPI hardware natively supports dual and quad modes, and these
modes are already enabled for SPI flash devices under spi-mem framework
in MTK SPI controller spi-mt65xx. However, other SPI devices, such as
touch panels, are limited to single mode because spi-mt65xx lacks SPI
mode argument parsing from SPI framework for these SPI devices outside
spi-mem framework.

This patch adds dual and quad mode support for these SPI devices by
introducing a new API, mtk_spi_set_nbits, for SPI mode argument parsing.

Signed-off-by: Tim Kuo <Tim.Kuo@mediatek.com>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Link: https://patch.msgid.link/20250917055839.500615-1-Tim.Kuo@mediatek.com
Signed-off-by: Mark Brown <broonie@kernel.org>