samples: openamp: Remove exclusive compile of openAMP

Add configuable shared memory address for openAMP samples. There is a
plan to add more platforms supported for openAMP in zephyr.

Each platform can specify the shared memory address and device by
device tree and add it's support in openAMP samples.

Signed-off-by: Karl Zhang <karl.zhang@linaro.org>
Signed-off-by: Kumar Gala <kumar.gala@linaro.org>