ext: open-amp: move open-amp down one dir

Move open-amp to be in ext/lib/ipc/open-amp/open-amp.  This allows us to
be Zephyr specific files and config like README (for import),
CMakeLists.txt file, Kconfig, etc in ext/lib/ipc/open-amp/ that don't
conflict with any files that might have the same name in
ext/lib/ipc/open-amp/open-amp.

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>