rpmsg: add a description field

Add a new description field to the rpmsg bus infrastructure
that can be passed onto the rpmsg client drivers for additional
information. The current rpmsg bus client drivers need to have
a fixed id_table for proper matching, this new field can allow
flexibility for the client drivers (eg: like creating unique
cdevs).

Signed-off-by: Ohad Ben-Cohen <ohad@wizery.com>
Signed-off-by: Suman Anna <s-anna@ti.com>