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.

Signed-off-by: Chris Lew <clew@codeaurora.org>
[bjorn: Altered how defaults are expressed]
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>