arm64: dts: Enable onboard SDHCI on msm8992

This enables SDHCI on the Nexus 5X as well creates common smd_rpm node
which can be shared between both 5X and 6P as per HW design.

Given the lack of documentation, only downstream code was used as a reference
and it eludes to the fact that 8994-rpm-regulator is common between both msm8992
& msm8994.   [ see msm.git branch: msm-angler-3.10-marshmallow-mr1, msm8992.dtsi]

At this early stage of development it makes sense for the 8994-rpm-regulator
to be common until data / documentation suggests otherwise.

Signed-off-by: Jeremy McNicoll <jeremymc@redhat.com>
Acked-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: Andy Gross <andy.gross@linaro.org>