spi: Move spi code from Documentation to tools

Jon Corbet requested this code moved with the last changeset,
https://lkml.org/lkml/2015/3/1/144

,
but the patch was not applied because it missed the Makefile.
Moved spidev_test, spidev_fdx and their Makefile infrastructure.

Signed-off-by: Joshua Clayton <stillcompiling@gmail.com>
Signed-off-by: Mark Brown <broonie@kernel.org>