dt-bindings: arm: rockchip: add rk3368 compatible string to pmu.yaml

Add the compatible for the pmu mfd on rk3368.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20210925090405.2601792-1-heiko@sntech.de