Move Zephyr example rpmsg_multi_services into Zephyr upstream

[glneo]

The Zephyr project seems open to examples and already has some for OpenAMP[0].

Examples here as a downstream will have the normal issues downstream projects tend to have (maintenance of keeping things in sync means we have these demos locked to a specific Zephyr version. Upstream they would be migrated with changes in Zephyr for any given version).

We should move the rpmsg_multi_services Zephyr example into upstream Zephyr and update the docs (examples/zephyr/README.md) to explain how to find and build that example in the Zephyr tree instead.

[0] https://github.com/zephyrproject-rtos/zephyr/tree/main/samples/subsys/ipc

[arnopo]

The rpmsg_multi_services sample relies on two things that prevent it from being mergeable in its current state in Zephyr:

• It requires a patch in Linux: Linux Patch, which allows the creation of rpmsg channels from the Linux side. This patch is mandatory for the demo3.2: create a new TTY channel

• The associated demo uses the rpmsg-utils tools.

If we address these points, in my opinion, we will have a sample very close to what is already available at Zephyr openamp_rsc_table.

This would mean:

• Remove Linux patch and capability to use the NS annoucement on Linux rpmsg endpoint creation.
• removing the rpmsg_multi_services sample (and possibly improving the Zephyr openamp_rsc_table sample)
• keeping the readme file explaining the demo using rpmsg-utils.