rpmsg: glink: Fix idr_lock from mutex to spinlock

The channel members lcids, rcids synchronised using the idr_lock is
accessed in both atomic/non-atomic contexts. The readers are not
currently synchronised.  That no correct, so add the readers as well
under the lock and use a spinlock.

Acked-by: Arun Kumar Neelakantam <aneela@codeaurora.org>
Signed-off-by: Sricharan R <sricharan@codeaurora.org>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>