rpmsg: pru: add a PRU RPMsg driver

commit fec62cdf254618a354aa1e87c5567c9f996d8a22 from
git://git.ti.com/ti-linux-kernel/ti-linux-kernel.git



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: various cleanups, forward-port to 5.10]
Signed-off-by: Suman Anna <s-anna@ti.com>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Xulin Sun <xulin.sun@windriver.com>