dt-bindings: arm: rockchip: add rk3568 compatible string to pmu.yaml

add "rockchip,rk3568-pmu", "syscon", "simple-mfd" for pmu nodes on a
rk3568 platform to pmu.ymal.

Signed-off-by: Liang Chen <cl@rock-chips.com>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20210624114719.1685-2-cl@rock-chips.com


Signed-off-by: Heiko Stuebner <heiko@sntech.de>