soc: qcom: glink_smem_native_xport: Add readback to write index

Writing data to DDR is not ensured to be completed even with addition
of data memory barrier. It is possible that CPU receive that write
instruction is completed but in reality data can be stuck in intermediate
buffer.

Readback of write index is added to ensure that remote side is sent
interrupt only after data is written on DDR.

CRs-Fixed: 2136645
Change-Id: I32c4967daf71991621b57421167e0ac8d38ce065
Signed-off-by: Dhoat Harpal <hdhoat@codeaurora.org>