dt-bindings: mfd: syscon: Add rk3368 QoS register compatible

Document rk3368 compatible for QoS registers.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Link: https://lore.kernel.org/r/20210925090405.2601792-2-heiko@sntech.de