The compatible strings below are already in use in the Rockchip
dtsi files, but were somehow never added to a document, so add

"rockchip,px30-pmu", "syscon", "simple-mfd"
"rockchip,rk3288-pmu", "syscon", "simple-mfd"
"rockchip,rk3399-pmu", "syscon", "simple-mfd"

for pmu nodes to pmu.yaml.
Signed-off-by: Johan Jonker <jbx6244@gmail.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20210417112952.8516-13-jbx6244@gmail.comSigned-off-by: Heiko Stuebner <heiko@sntech.de>