dt-bindings: mailbox: qcom,apcs-kpss-global: correct expected clocks for fallbacks

Commit 1e9cb7e0 ("dt-bindings: mailbox: qcom,apcs-kpss-global: use
fallbacks") and commit 34d8775a ("dt-bindings: mailbox:
qcom,apcs-kpss-global: use fallbacks for few variants") added fallbacks
to few existing compatibles.  Neither devices with these existing
compatibles nor devices using fallbacks alone, have clocks, so the
"if:then:" block defining this constrain should be written as
"contains:".

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Acked-by: Rob Herring (Arm) <robh@kernel.org>
Signed-off-by: Jassi Brar <jassisinghbrar@gmail.com>