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.

Change-Id: Ie7e1fd03320a860faeddfadb8a1c7661913c33a6
Signed-off-by: Sricharan R <sricharan@codeaurora.org>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
[clew@codeaurora.org: fix GFP_KERNEL use in spinlock context]
Signed-off-by: Chris Lew <clew@codeaurora.org>