dt-bindings: mfd: syscon: Add rk3568 QoS register compatible

Document rk3568 compatible for QoS registers.

Signed-off-by: Liang Chen <cl@rock-chips.com>
Acked-by: Rob Herring <robh@kernel.org>
Acked-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Lee Jones <lee.jones@linaro.org>