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/tools/spi/Makefile b/tools/spi/Makefile
new file mode 100644
index 0000000..cd0db62
--- /dev/null
+++ b/tools/spi/Makefile
@@ -0,0 +1,4 @@
+all: spidev_test spidev_fdx
+
+clean:
+	$(RM) spidev_test spidev_fdx