soc: qcom: smem: Use write-combine remap for SMEM

Mapping the SMEM region as write combine makes the contiguous writes
in SMD perform better and also allows us to do unaligned read and writes
on ARM64.

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