arm64: dts: msm8996: Add rmtfs sharedmem node

A 2MB shared memory region is used on MSM8996 for exchanging sector data
in rmtfs. Add this chunk of reserved memory now that we have the
rmtfs-mem compatible to describe it and its memory protection
properties.

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