spi: Document Octal mode as valid SPI bus width

SPI core supports Octal SPI controllers which have 8 IO lines.
Therefore document 8 as a valid option for spi-tx{rx}-bus-width

Signed-off-by: Vignesh Raghavendra <vigneshr@ti.com>
Link: https://lore.kernel.org/r/20200102102118.23318-1-vigneshr@ti.com


Signed-off-by: Mark Brown <broonie@kernel.org>