spi: spi-cavium-thunderx: flag controller as half duplex

spi: spi-cavium-thunderx: flag controller as half duplex

The OcteonTX (TX1/ThunderX) SPI controller does not support full
duplex transactions. Set the appropriate flag such that the spi
core will return -EINVAL on such transactions requested by chip
drivers.

This is an RFC as I need someone from Marvell/Cavium to confirm
if this driver is used for other silicon that does support
full duplex transfers (in which case we will need to identify
that we are running on the ThunderX arch before setting the flag).

Signed-off-by: Tim Harvey <tharvey@gateworks.com>
Cc: Robert Richter <rrichter@marvell.com>
Link: https://lore.kernel.org/r/1590680799-5640-1-git-send-email-tharvey@gateworks.com
Signed-off-by: Mark Brown <broonie@kernel.org>