dt-bindings: spi: convert rockchip spi bindings to yaml

Current dts files with 'spi' nodes are manually verified.
In order to automate this process spi-rockchip.txt
has to be converted to yaml. In the new setup
spi-rockchip.yaml will inherit properties from
spi-controller.yaml.

Add document to MAINTAINERS.

Also rk3188.dtsi, rk3288.dtsi, rk3368.dtsi and rk3399.dtsi
use an extra fallback string, so change this in the documentation.

Changed:
"rockchip,rk3188-spi", "rockchip,rk3066-spi"
"rockchip,rk3288-spi", "rockchip,rk3066-spi"
"rockchip,rk3368-spi", "rockchip,rk3066-spi"
"rockchip,rk3399-spi", "rockchip,rk3066-spi"

Signed-off-by: Johan Jonker <jbx6244@gmail.com>
Link: https://lore.kernel.org/r/20200304184203.9548-1-jbx6244@gmail.com
Signed-off-by: Mark Brown <broonie@kernel.org>