There is an ever growing list of compatibles in the smd-rpm.c driver.
A fallback compatible would help here but would still require keeping
the current list around for backwards compatibility.

As an alternative, let's switch the driver to match the rpmsg_device_id
instead, which is always "rpm_requests" on all platforms. Add a check
to ensure that there is a device tree node defined for the device since
otherwise the of_platform_populate() call will operate on the root node (/).

Similar approaches with matching rpmsg_device_id are already used in
qcom_sysmon, qcom_glink_ssr, qrtr, and rpmsg_wwan_ctrl.

Tested-by: Konrad Dybcio <konrad.dybcio@linaro.org> # SM6375 (G-Link)
Reviewed-by: Konrad Dybcio <konrad.dybcio@linaro.org>
Signed-off-by: Stephan Gerhold <stephan@gerhold.net>
Link: https://lore.kernel.org/r/20230531-rpm-rproc-v3-4-a07dcdefd918@gerhold.netSigned-off-by: Bjorn Andersson <andersson@kernel.org>