arm64: dts: qcom: msm8996: Unify smp2p naming

Rename smp2p-modem to smp2p-mpss, and make the subnode labels of smp2p_adsp
and smp2p_slpi follow the <name>_smp2p_<out/in> layout.
Also move smp2p_slpi_out above smp2p_slpi_in to make it match mpss and adsp
where master-kernel is the first subnode.

This patch brings no functional change.

Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>