spi: s3c64xx: document the clocks and the clock-name property

These two properties were not documented but used in the spi
dts. Add the related documentation.

Suggested-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Signed-off-by: Andi Shyti <andi.shyti@samsung.com>
Reviewed-by: Michael Turquette <mturquette@baylibre.com>
Reviewed-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Signed-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/spi/spi-samsung.txt

@@ -24,6 +24,15 @@ Required SoC Specific Properties:
 - dma-names: Names for the dma channels. There must be at least one channel
   named "tx" for transmit and named "rx" for receive.
 
+- clocks: specifies the clock IDs provided to the SPI controller; they are
+  required for interacting with the controller itself, for synchronizing the bus
+  and as I/O clock (the latter is required by exynos5433 and exynos7).
+
+- clock-names: string names of the clocks in the 'clocks' property; for all the
+  the devices the names must be "spi", "spi_busclkN" (where N is determined by
+  "samsung,spi-src-clk"), while Exynos5433 should specify a third clock
+  "spi_ioclk" for the I/O clock.
+
 Required Board Specific Properties:
 
 - #address-cells: should be 1.