dt-bindings: mailbox: qcom: correct clocks for IPQ6018 and IPQ8074

IPQ6018 APSS driver is registered by APCS as they share the same register
space, and it uses "pll" and "xo" as inputs.

Correct the allowed clocks for IPQ6018 and IPQ8074 as they share the same
driver to allow "pll" and "xo" as clock-names.

Signed-off-by: Robert Marko <robimarko@gmail.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>