rpmsg: char: Add possibility to use default endpoint of the rpmsg device

Current implementation create/destroy a new endpoint on each
rpmsg_eptdev_open/rpmsg_eptdev_release calls.

For a rpmsg device created by the NS announcement a default endpoint is created.
In this case we have to reuse the default rpmsg device endpoint associated to
the channel instead of creating a new one.

This patch prepares the introduction of a rpmsg channel device for the
char device. The rpmsg channel device will require a default endpoint to
communicate to the remote processor.

Add the default_ept field in rpmsg_eptdev structure.This pointer
determines the behavior on rpmsg_eptdev_open and rpmsg_eptdev_release call.

- If default_ept == NULL:
  Use the legacy behavior by creating a new endpoint each time
  rpmsg_eptdev_open is called and release it when rpmsg_eptdev_release
  is called on /dev/rpmsgX device open/close.

- If default_ept is set:
  use the rpmsg device default endpoint for the communication.

Add protection in rpmsg_eptdev_ioctl to prevent to destroy a default endpoint.

Signed-off-by: Arnaud Pouliquen <arnaud.pouliquen@foss.st.com>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Link: https://lore.kernel.org/r/20220124102524.295783-10-arnaud.pouliquen@foss.st.com