spi: octeon: Add ThunderX driver

Add ThunderX SPI driver using the shared part from the Octeon
driver. The main difference of the ThunderX driver is that it
is a PCI device so probing is different. The system clock settings
can be specified in device tree.

Signed-off-by: Jan Glauber <jglauber@cavium.com>
Signed-off-by: Mark Brown <broonie@kernel.org>