rpmsg: virtio: EPOLLOUT support

rpmsg: virtio: EPOLLOUT support

Previously, polling an rpmsg endpoint (e.g. /dev/ttyRPMSGx) would
generate EPOLLIN events but no EPOLLOUT events.

Unfortunately, poll support means that we can no longer disable
tx-complete interrupts as there is no way to know whether a poller is
waiting in sendq, so we always need notifications.

Signed-off-by: Patrick Oppenlander <patrick.oppenlander@gmail.com>
Link: https://lore.kernel.org/r/20251022032817.320378-1-patrick.oppenlander@gmail.com
Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>