soc: qcom: rmtfs: Add support for mmap functionality

This change adds mmap functionality to rmtfs_mem driver.
Userspace application can map the address and use this
mapped address directly as buffer for read/write call to disk.
and avoid the read/write call to the shared path to copy the
buffer to userspace application.

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