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>
diff --git a/Documentation/Makefile b/Documentation/Makefile
index bc05482..1207d79 100644
--- a/Documentation/Makefile
+++ b/Documentation/Makefile
@@ -1,4 +1,4 @@
 subdir-y := accounting auxdisplay blackfin connector \
 	filesystems filesystems ia64 laptops mic misc-devices \
-	networking pcmcia prctl ptp spi timers vDSO video4linux \
+	networking pcmcia prctl ptp timers vDSO video4linux \
 	watchdog