remoteproc: pru: Add support for virtio rpmsg stack

commit 52e18bc6bcbd67c3b822ced4d06f8e8b5f3fad1f from
git://git.ti.com/ti-linux-kernel/ti-linux-kernel.git



The PRU remoteproc driver has been enhanced to support the optional
rpmsg stack using the virtio-ring based communication transport
between MPU and a PRU core. This provides support to any firmware
images supporting the virtio devices.

The virtio-ring signalling support is provided through two PRU system
events - one event used in each direction for kicking from one processor
and receiving notification on the other processor. It provides an
uniform solution across all the OMAP, Keystone and Davinci architectures.
The virtio-ring based communication requires the corresponding firmware
support though.

Because the vring irq mappings described in DT may conflict with
some PRU client irq mappings described in DT, move part responsible for
gathering vring irq to pru_rproc_start stage and perform it only if
rvdevs are present.

Signed-off-by: Suman Anna <s-anna@ti.com>
Co-developed-by: Grzegorz Jaszczyk <grzegorz.jaszczyk@linaro.org>
[grzegorz.jaszczyk@linaro.org: Gather vring irq only when rvdevs
are present]
Signed-off-by: Grzegorz Jaszczyk <grzegorz.jaszczyk@linaro.org>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Xulin Sun <xulin.sun@windriver.com>