dt-bindings: spi: spi-rockchip: add description for rk3308

The description below is already in use for rk3308.dtsi,
but was somehow never added to a document, so add
"rockchip,rk3308-spi", "rockchip,rk3066-spi"
for spi nodes on a rk3308 platform to spi-rockchip.yaml.

Signed-off-by: Johan Jonker <jbx6244@gmail.com>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20200304184203.9548-2-jbx6244@gmail.com
Signed-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/spi/spi-rockchip.yaml

@@ -29,6 +29,7 @@ properties:
             - rockchip,px30-spi
             - rockchip,rk3188-spi
             - rockchip,rk3288-spi
+            - rockchip,rk3308-spi
             - rockchip,rk3368-spi
             - rockchip,rk3399-spi
           - const: rockchip,rk3066-spi