rpmsg: glink: Add support to preallocate intents

The base intents prequeued during channel creation may not satisfy a
channel's throughput requirement. Add support for intents dt-binding to
allow channels to specify the size and amount of intents to prequeue
during endpoint announcement.

Change-Id: Ic0465e8afe69874429328d65e393e29cb89d3722
Signed-off-by: Chris Lew <clew@codeaurora.org>