arm64: dts: qcom: msm8916: Drop standalone smem node

SMEM can now be described directly in the reserved-memory.
This is mainly meant for newer SoCs where there is only one SMEM
region. However, even on older SoCs like MSM8916 there is clearly one
main SMEM region (described by "memory-region") that holds the
smem_header and one special extra region used only for data of the
RPM ("qcom,rpm-msg-ram").

The definition in reserved-memory also looks cleaner for older SoCs,
so make use of that in MSM8916 as well.

Signed-off-by: Stephan Gerhold <stephan@gerhold.net>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Link: https://lore.kernel.org/r/20211018110009.30837-2-stephan@gerhold.net