modem/msm-modem: rename msm-modem-mainline to msm-modem-rpmsg (MR 1356)

Newer Qualcomm SoCs (e.g. SDM845. MSM8953) seem to use a different
method to talk to the modem. There are no longer separate rpmsg/SMD
channels for QMI messags, instead the modem is also available through QRTR.

On these newer SoCs, installing msm-modem-mainline is pointless,
because the created /dev/modem device will not actually allow
communicating with the modem. However, you still need the part from
the base package (msm-modem), which installs+enables rmtfs.

To avoid confusion, rename the msm-modem-mainline subpackage to
msm-modem-rpmsg. Install only "msm-modem" on SDM845 because that
likely does not allow communication with the modem through RPMSG
anyway.