arm64: dts: qcom: sdm845: Define rmtfs memory

Define the rmtfs memory node. As the memory region specified in version
10 of the memory map is only 1MB a chunk of unallocated memory is
chosen.

Tested-by: Sibi Sankar <sibis@codeaurora.org>
Reviewed-by: Sibi Sankar <sibis@codeaurora.org>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: Andy Gross <agross@kernel.org>