rpmsg: pru: add a PRU RPMsg driver

An RPMsg driver that exposes interfaces to user space, to
allow applications to communicate with the PRU processors
on available TI SoCs has been added. This is restricted to
SoCs that have the PRUSS remoteproc support.

Signed-off-by: Jason Reeder <jreeder@ti.com>
[s-anna@ti.com: rename some variables, checkpatch & sparse fixes]
Signed-off-by: Suman Anna <s-anna@ti.com>