arm64: dts: qcom: msm8916: Add entry-method property for the idle-states node

The idle-states binding documentation[1] mentions that the
'entry-method' property is required on 64-bit platforms and must be set
to "psci".

[1] Documentation/devicetree/bindings/arm/idle-states.txt (see
idle-states node)

Acked-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Reviewed-by: Niklas Cassel <niklas.cassel@linaro.org>
Signed-off-by: Amit Kucheria <amit.kucheria@linaro.org>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: Andy Gross <agross@kernel.org>