dt-bindings: arm: rockchip: add rk5388 compatible string to pmu.yaml

Add the compatible for the pmu mfd on rk3588.

Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>
Link: https://lore.kernel.org/r/20220906143825.199089-2-sebastian.reichel@collabora.com


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