spi: add OpenCores tiny SPI driver

This patch adds support of OpenCores tiny SPI driver.

http://opencores.org/project,tiny_spi

Signed-off-by: Thomas Chou <thomas@wytron.com.tw>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>

Documentation/devicetree/bindings/spi/spi_oc_tiny.txt

@@ -0,0 +1,12 @@
+OpenCores tiny SPI
+
+Required properties:
+- compatible : should be "opencores,tiny-spi-rtlsvn2".
+- gpios : should specify GPIOs used for chipselect.
+Optional properties:
+- clock-frequency : input clock frequency to the core.
+- baud-width: width, in bits, of the programmable divider used to scale
+	the input clock to SCLK.
+
+The clock-frequency and baud-width properties are needed only if the divider
+is programmable. They are not needed if the divider is fixed.