soc: qcom: rmtfs-mem: Add support for assigning memory to remote

On some platform the remote processor's memory map is not statically
configured in TrustZone, so each memory region that is to be accessed by
the remote needs a call into TrustZone to set up the remote's
permissions.

Implement this for the rmtfs memory driver, to give the modem on 8996
access to the shared file system buffers.

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