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>