dt-bindings: arm: cpus: Document "qcom,msm8909-smp" enable-method

MSM8909 is a fairly old 32-bit SoC without PSCI support, so the
additional CPU cores need to be initialized with a custom enable-method.
Fortunately it works just like on MSM8226 and MSM8916 so just add
an additional compatible as alias to the DT schema.

Signed-off-by: Stephan Gerhold <stephan.gerhold@kernkonzept.com>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Link: https://lore.kernel.org/r/20220705143523.3390944-8-stephan.gerhold@kernkonzept.com