rpmsg: glink: rpm: Wrap driver context

As with the SMEM driver update, wrap the RPM context in a struct to
facilitate the upcoming changes of moving IRQ and mailbox registration
to the driver.

Reviewed-by: Chris Lew <quic_clew@quicinc.com>
Signed-off-by: Bjorn Andersson <quic_bjorande@quicinc.com>
Signed-off-by: Bjorn Andersson <andersson@kernel.org>
Link: https://lore.kernel.org/r/20230213155215.1237059-4-quic_bjorande@quicinc.com