soc: qcom: rmtfs-mem: Add class to enable uevents

Currently the qcom_rmtfs_memN devices are entirely invisible to the udev world.
Add a class to the rmtfs device so that uevents fire when the device is added.

Reviewed-by: Brian Norris <briannorris@chromium.org>
Reviewed-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: Evan Green <evgreen@chromium.org>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: Andy Gross <andy.gross@linaro.org>