arm64: dts: msm8916: Mark rmtfs node as qcom, rmtfs-mem compatible

Now that we have a binding defined for the shared file system memory use
this to describe the rmtfs memory region.

Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: Andy Gross <andy.gross@linaro.org>