arm64: dts: msm8996: Add SMEM reserve-memory node

Add DT node to carveout memory for shared memory region.

Reviewed-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: Sarangdhar Joshi <spjoshi@codeaurora.org>
Signed-off-by: Andy Gross <andy.gross@linaro.org>